Serves, with "on" (5)
I believe the answer is:
waits
'serves with on' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for waits that I've seen before include "Night-time Yuletide singers" , "Lingers" , "Stays" , "Stands by - carollers" , "Carol singers" .)