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