That man breaks first commandment (6)
I believe the answer is:
behest
'commandment' is the definition.
(thesaurus)
'that man breaks first' is the wordplay.
'that man' becomes 'he' (he refers to a man).
'breaks' indicates putting letters inside (inserted letters break into the word).
'first' becomes 'best' (similar in meaning).
'he' placed within 'best' is 'BEHEST'.
(Other definitions for behest that I've seen before include "(At someone's) command" , "Command - Thebes (anag)" , "Command, bidding" , "Order" .)