Instruments become before long in low pitch (8)
I believe the answer is:
bassoons
'instruments' is the definition.
(bassoon is a kind of instrument)
'before long in low pitch' is the wordplay.
'before long' becomes 'soon' (synonyms).
'in' is an insertion indicator.
'low pitch' becomes 'bass' (bass is a kind of low pitch).
'soon' inserted into 'bass' is 'BASSOONS'.
'become' acts as a link.
(Other definitions for bassoons that I've seen before include "Large woodwind instruments" , "Tenors of the oboe family" , "Playing them" , "Tenor instruments, in the oboe family" , "Double-reed instruments" .)