Indispensable aide (5-4,3)
I believe the answer is:
right-hand man
'indispensable aide' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for right-hand man that I've seen before include "Indispensable assistant" , "Reliable assistant" , "One's most valuable assistant" , "Most trusted assistant" , "Chief assistant; major domo" .)