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