Firm's first man takes on worker (7)
I believe the answer is:
adamant
'firm's' is the definition.
(thesaurus)
'first man takes on worker' is the wordplay.
'first man' becomes 'adam' (I've seen this before).
'takes on' is a charade indicator (letters next to each other).
'worker' becomes 'ant' (worker is a type of ant).
'adam'+'ant'='ADAMANT'
(Other definitions for adamant that I've seen before include "popstar" , "Definite and unyielding" , "Unshakable (in an opinion)" , "Fabulous rock" , "Hard and unrelenting" .)