Benefit of a mask in speech (5)
I believe the answer is:
avail
'benefit' is the definition.
(I've seen this in another clue)
'a mask in speech' is the wordplay.
'mask' becomes 'veil' (I've seen this in another clue).
'in speech' indicates a 'sounds like' (homophone) clue.
'veil' is a homophone of 'vail'.
'a'+'vail'='AVAIL'
'of' acts as a link.
(Other definitions for avail that I've seen before include "Serve a purpose" , "Provide help; profit" , "Use to one's advantage" , "Help, benefit" , "Be of value to" .)