Help the last assistant (4)
I believe the answer is:
aide
'assistant' is the definition.
(aide is a kind of assistant)
'help the last' is the wordplay.
'help' becomes 'aid' (synonyms).
'last' indicates one should take the final letters.
The final letter of 'the' is 'e'.
'aid'+'e'='AIDE'
(Other definitions for aide that I've seen before include "Idea (anag)" , "Adviser or assistant" , "Personal assistant in the military" , "auxiliary" , "Somebody helping" .)