Top man's in command (6)
I believe the answer is:
behest
'command' is the definition.
(behest is a kind of command)
'top man's in' is the wordplay.
'top' becomes 'best' ('best' can be similar in meaning to 'top'**).
'man' becomes 'he'.
'in' indicates putting letters inside.
'best' placed around 'he' is 'BEHEST'.
(Other definitions for behest that I've seen before include "Command, bidding" , "Order" , "(At someone's) command" , "Command - Thebes (anag)" .)