Completely stuck in the middle - my "vocabulary" just aint good enough for this!!.....
12a Group fuck joiner (8) ????C?E?
13a Windy Caribbean ox did fart! (6,8) ?A?B?N / ?????I?E looks like an anagram of caribbean ox did ? possibly carbon ...
15a Ten pinter in Will & Grace (7,7) ??R???? / ?A?L?N? grace might be darling?