Breaks wind quietly inside (4)
I believe the answer is:
gaps
'breaks' is the definition.
('gap' can be a synonym of 'break')
'wind quietly inside' is the wordplay.
'wind' becomes 'gas' (synonyms).
'quietly' becomes 'p' (p for piano and quiet).
'inside' is an insertion indicator.
'gas' enclosing 'p' is 'GAPS'.
(Other definitions for gaps that I've seen before include "Breaks or openings as in hedge or fence" , "Breaks, spaces" .)