Useful assistant on end of party (5)
I believe the answer is:
handy
'useful' is the definition.
(handy can mean useful or convenient)
'assistant on end of party' is the wordplay.
'assistant' becomes 'hand' (hand can mean a worker or employee).
'on' is a charade indicator (letters next to each other).
'end of' indicates one should take the final letters.
The final letter of 'party' is 'y'.
'hand'+'y'='HANDY'
(Other definitions for handy that I've seen before include "Convenient summary" , "Useful - competent" , "Local - nearby" , "Useful - within reach" , "Useful and convenient" .)