Group of musicians adjust private parts (6)
I believe the answer is:
'group of musicians' is the definition.
'adjust private parts' is the wordplay.
'adjust' becomes 'set' (setting is a kind of adjusting).
'private' becomes 'pte'.
'parts' means one lot of letters goes inside another (in the sense of cuts or separates).
'set' placed around 'pte' is 'SEPTET'.
(Other definitions for septet that I've seen before include "G7" , "Seven-piece band" , "Group of seven players" , "Group of seven performers or singers" , "Several players" .)