Be of use to (5)
I believe the answer is:
avail
'be of use to' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for avail that I've seen before include "Use to one's advantage" , "(Of no) use or benefit" , "Be of help or advantage" , "Medal (informal)" , "Be of service to" .)