Group about to fix old instrument (6)
I believe the answer is:
spinet
'old instrument' is the definition.
(I've seen this before)
'group about to fix' is the wordplay.
'group' becomes 'set' (set is a kind of group).
'about' is an insertion indicator.
'to fix' becomes 'pin' (pinning is a kind of fixing).
'set' going around 'pin' is 'SPINET'.
(Other definitions for spinet that I've seen before include "Small 12" , "Type of early harpsichord" , "Harpsichord-like keyboard instrument" , "Early keyboard instrument" , "Step in with old musical instrument" .)