A man now is boss (4)
I believe the answer is:
head
'is boss' is the definition.
(synonyms)
'a man now' is the wordplay.
'a man' becomes 'he'.
'now' becomes 'ad' (I've seen this in other clues).
'he'+'ad'='HEAD'
(Other definitions for head that I've seen before include "Leader; front" , "Noggin" , "John on board" , "Lead - school principal" , "Be in charge of; (come to) crisis" .)