A face covering said to be of use (5)
I believe the answer is:
avail
'be of use' is the definition.
(I've seen this before)
'a face covering said' is the wordplay.
'face covering' becomes 'veil' (I've seen this before).
'said' shows a homophone (sound like).
'veil' is a homophone of 'vail'.
'a'+'vail'='AVAIL'
'to' is the link.
(Other definitions for avail that I've seen before include "Use to one's advantage" , "Be useful" , "Be of service to" , "Provide help; profit" , "See 3" .)