Firm's worker overseen by leading man (7)
I believe the answer is:
adamant
'firm's' is the definition.
(adamant can mean firm or determined)
'worker overseen by leading man' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'overseen by' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'leading man' becomes 'adam' (man's name. I am not sure about the 'leading' bit.).
'ant' put after 'adam' is 'ADAMANT'.
Can you help me to learn more?
(Other definitions for adamant that I've seen before include "Determined and unyielding to requests" , "Definite and unyielding" , "Legendary rock" , "Refusing to change one's mind" , "6a [PRINCESS IDA] 's fortress < for unyielding" .)