One, a devil in disguise, helped (7)
I believe the answer is:
availed
'helped' is the definition.
(availing is a kind of helping)
'one a devil in disguise' is the wordplay.
'one' becomes 'a'.
'in disguise' indicates anagramming the letters.
'a'+'devil'='adevil'
'adevil' with letters rearranged gives 'vailed'.
'a'+'vailed'='AVAILED'
(Other definitions for availed that I've seen before include "Used, put into service" , "Profited from" , "Helped" , "Used to one's advantage" .)