Firm punch nails cheating brother (5)
I believe the answer is:
jacob
'cheating brother' is the definition.
(Jacob cheated his brother Esau in the Bible)
'firm punch nails' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'punch' becomes 'jab' (jab is a kind of punch).
'nails' is an insertion indicator (inserted letters are captured or nailed).
'co' placed inside 'jab' is 'JACOB'.
(Other definitions for jacob that I've seen before include "Fellow" , "He saw ladder" , "-- Rees Mogg, MP" , "Name of patriarch" , "He had a vision of a ladder into heaven with angels going up and down it" .)