A cover said to be of use (5)
I believe the answer is:
avail
'use' is the definition.
(availing is a kind of using)
'a cover said to be' is the wordplay.
'cover' becomes 'veil' (I've seen this in another clue).
'said to be' shows a homophone (sound like).
'veil' is a homophone of 'vail'.
'a'+'vail'='AVAIL'
'of' is the link.
(Other definitions for avail that I've seen before include "Be of service to" , "(Of no) use or benefit" , "Be of help or advantage" , "Be of value to" , "See 3" .)