He is in highest command (6)
I believe the answer is:
behest
'command' is the definition.
(I know that behest is a type of command)
'he is in highest' is the wordplay.
'is in' is an insertion indicator.
'highest' becomes 'best'.
'he' inserted into 'best' is 'BEHEST'.
(Other definitions for behest that I've seen before include "(At someone's) command" , "Command - Thebes (anag)" , "Order" , "Command, bidding" .)