Use ace disguise, we hear (5)
I believe the answer is:
avail
'use' is the definition.
(availing is a kind of using)
'ace disguise we hear' is the wordplay.
'ace' becomes 'a' (abbreviation).
'disguise' becomes 'veil' (I've seen this before).
'we hear' indicates a 'sounds like' (homophone) clue.
'veil' sounds like 'vail'.
'a'+'vail'='AVAIL'
(Other definitions for avail that I've seen before include "Be useful" , "Provide help; profit" , "Use to one's advantage" , "Be of help or advantage" , "Be of service to" .)