He is bound by supreme command (6)
I believe the answer is:
behest
'command' is the definition.
(behest is a kind of command)
'he is bound by supreme' is the wordplay.
'is bound by' means one lot of letters goes inside another (I've seen this in other clues).
'supreme' becomes 'best' (I've seen this before).
'he' going into 'best' is 'BEHEST'.
(Other definitions for behest that I've seen before include "Command, bidding" , "(At someone's) command" , "Command - Thebes (anag)" , "Order" .)