Tries from Frank Ryan and Andy Cummins both converted by Niall Leahy left the score at 14-3 at half-time to the home side. Young Munster fought back to score a converted try in the second half but could not score again as Clanwilliam prevailed.
In the other game in group A Tralee defeated Kinsale 24-13 in their clash in Kerry.
Last season's Under 17 Pan Munster champions, Clonakilty, continued where they left off last season when they kicked off their Under 18 campaign on Saturday with a 36-0 victory over Galbally. While in the other group 2 clash Bruff were 31-22 victors over Highfield in Kilballyowen Park in a very exciting game which saw the home side lead 23-3 at half time before the Cork side came back strong to bring the score to 23-15 going into the final quarter of the game. An enthralling final quarter saw both sides score tries as they pushed for the win. Bruff will be thankful for the boot of Paul Downes whose 14 points were pivotal in his side's win.
Cork Constitution narrowly won their Cork derby clash in Group 3 defeating Cobh Pirates 10-8 while Old Crescent defeated Nenagh Ormond 19-7 in a clash played under lights in Rosbrien on Sunday evening.
Sunday's Well took the points from their trip to Killarney with a 14-0 victory while East Munster League champions Waterpark were victorious in Ballinakill when they defeated the travelling Newcastle West side 17-10.
At Under 16 level Bantry Bay's first game in the Pan Munster Competition resulted in success on Saturday when they defeated Sunday's Well 11-3 in Bantry. The away side started the stronger and only superb defence from Bantry kept Sunday's Well from scoring the opening try of the game. Sunday's Well eventually scored a penalty to leave the score at 3-0 to the visitors at half time. Bantry began to dominate in the second half and the pressure they were putting on the visitors resulted in two penalties both of which were converted to move the hosts into a 6-3 lead 10 minutes into the second half. Bantry Bay then went on to score an unconverted try to clinch the victory. Waterpark were 13-0 victors over Shannon in the other game in this pool.
Castleisland defeated Dolphin 22-18 in their group 2 clash in Mahon on Saturday while East Munster League champions Kilfeacle lead the way in group 3 following their 19-7 victory over Kilrush.
Tralee were big winners over Muskerry in their pool 4 clash on Saturday while Dungarvan were victorious against Old Crescent in the West Waterford venue.
Pan Munster Under 18 League Results
Group 1: Clanwilliam 14 Young Munster 10, Tralee 24 Kinsale 13;
Group 2: Bruff 31 Highfield 22, Clonakilty 36 Galbally 0;
Group 3: Cork Constitution 10 Cobh Pirates 8, Old Crescent 19 Nenagh Ormond 7;
Group 4: Killarney 0 Sunday's Well 14, Waterpark 17 Newcastle West 10;
Pan Munster Under 16 League Results
Group 1: Bantry Bay 11 - 3 Sunday's Well, Shannon 0 - 13 Waterpark
Group 2: Dolphin 18 - 22 Castleisland, Bruff v Fethard
Group 3: Kilfeacle 19 - 7 Kilrush, Killorglin v Clonakilty
Group 4: Tralee 47 - 0 Muskerry, Dungarvan defeated Old Crescent
Next weekend's fixtures
Pan Munster Under 18 League
Group 1: Tralee v Clanwilliam, Kinsale v Young Munster
Group 2: Highfield v Galbally, Bruff v Clonakilty
Group 3: Cobh Pirates v Nenagh Ormond, Cork Constitution v Old Crescent
Group 4: Waterpark v Killarney, Newcastle West v Sunday's Well
Pan Munster Under 16 League
Group 1: Waterpark v Bantry Bay, Shannon v Sundays Well
Group 2: Castleisland v Fethard, Dolphin v Bruff
Group 3: Kilfeacle v Killorglin, Clonakilty v Kilrush
Group 4: Tralee v Dungarvan, Muskerry v Old Crescent