NCO’s firm scowl overwhelming cloth worker (6,8)
I believe the answer is:
colour sergeant
'nco's' is the definition.
(type of non-commissioned officer)
'firm scowl overwhelming cloth worker' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'scowl' becomes 'lour' (lour can mean a scowl or frown).
'overwhelming' means one lot of letters go next to another (in a down clue, some letters go over others).
'cloth' becomes 'serge' (serge is a kind of cloth).
'worker' becomes 'ant' (worker is a type of ant).
'co'+'lour'+'serge'+'ant'='COLOUR-SERGEANT'
(Other definitions for colour sergeant that I've seen before include "army officer" , "Senior NCO" .)