A curtain said to be useful (5)
I believe the answer is:
avail
'be useful' is the definition.
(I've seen this before)
'a curtain said' is the wordplay.
'curtain' becomes 'veil' (I've seen this before).
'said' indicates a 'sounds like' (homophone) clue.
'veil' sounds like 'vail'.
'a'+'vail'='AVAIL'
'to' acts as a link.
(Other definitions for avail that I've seen before include "Provide help; profit" , "Be of help or advantage" , "Be of service to" , "Help, benefit" , "Make use, take advantage" .)