A disguise is said to be an advantage (5)
I believe the answer is:
avail
'advantage' is the definition.
(I've seen this in another clue)
'a disguise is said to be an' is the wordplay.
'a disguise' becomes 'veil' (I've seen this before).
'is said' shows a homophone (sound like).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'be an' becomes 'a'.
'veil' is a homophone of 'vail'.
'vail' put after 'a' is 'AVAIL'.
(Other definitions for avail that I've seen before include "(Of no) use or benefit" , "Be useful" , "Use to one's advantage" , "See 3" , "Be of service to" .)