I've got three "answers" which I can't parse.
21a Party after supporting seat (8) B?C?S?D? I think it's BACKSIDE but the rationale seems quite thin.
23a Lovely drop, in parts (6) ?I?I?E I'm thinking DIVINE but can't see it.
13d Wrap filled with what? Some chocolate inside (6,4) ?E?I?D ?A?S I'm thinking BEHIND BARS but apart from "some chocolate" = BAR not getting very far.