Use a mask, we are told (5)
I believe the answer is:
avail
'use' is the definition.
(availing is a kind of using)
'a mask we are told' is the wordplay.
'mask' becomes 'veil' (I've seen this before).
'we are told' shows a homophone (sound like).
'veil' sounds like 'vail'.
'a'+'vail'='AVAIL'
(Other definitions for avail that I've seen before include "Be useful" , "Use to one's advantage" , "(Of no) use or benefit" , "Medal (informal)" , "See 3" .)