Be of use (5)
'be of use' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for serve that I've seen before include "Distribute" , "Work for, be of use to" , "deal" , "Be useful to" , "Answer the purpose" .)
'be of use' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for avail that I've seen before include "Make use, take advantage" , "(Of no) use or benefit" , "Provide help; profit" , "Medal (informal)" , "Help, benefit" .)