Struggling with last two:
18a "What's behind bar? Tango" (4): I've got B ? T ?
and can only come up with BUTT which you could find behind a bar. I can see the T for Tango but not sure about the BUT for "what's".
26a "Weak trombone initially repelled a musical group perhaps" (5). I've got T ? O ? P and if right can only come up with TROMP as a sort of weak trombone, but am stumped on the musical group
Any help much appreciated