Prepare to engage private music group (6)
I believe the answer is:
septet
'music group' is the definition.
(group of seven musicians)
'prepare to engage private' is the wordplay.
'prepare' becomes 'set' (synonyms).
'to engage' is an insertion indicator (some letters get engaged in others).
'private' becomes 'pte'.
'set' going around 'pte' is 'SEPTET'.
(Other definitions for septet that I've seen before include "Band of seven players" , "A group of seven, say singers" , "Group of seven performers or singers" , "A set of seven" , "Musical group" .)