Breathe without sound (6)
I believe the answer is:
exhale
'breathe' is the definition.
(exhaling is a kind of breathing)
'without sound' is the wordplay.
'without' becomes 'ex' (I've seen this in other clues).
'sound' becomes 'hale' ('hale' can be a synonym of 'sound'**).
'ex'+'hale'='EXHALE'
(Other definitions for exhale that I've seen before include "send forth wind" , "Emit breath" , "Joint of chicken" , "produce emissions" , "Discharge" .)