Instruments before long interrupting singer (8)
I believe the answer is:
bassoons
'instruments' is the definition.
(bassoon is a kind of instrument)
'before long interrupting singer' is the wordplay.
'before long' becomes 'soon' (I've seen this before).
'interrupting' is an insertion indicator.
'singer' becomes 'bass' (bass is a kind of singer).
'soon' going within 'bass' is 'BASSOONS'.
(Other definitions for bassoons that I've seen before include "Woodwind instruments with a very low sound" , "Large woodwind instruments" , "Tenor instruments, in the oboe family" , "Double-reed instruments" , "Playing them" .)