Cut to protect small firm's capital (6)
I believe the answer is:
moscow
'capital' is the definition.
(capital of Russia)
'cut to protect small firm's' is the wordplay.
'cut' becomes 'mow' (as in mowing a lawn).
'to protect' means one lot of letters goes inside another (inserted letters are held or protected).
'small' becomes 's' (abbreviation - e.g. clothes size).
'firm' becomes 'co' (a firm is a company).
's'+'co'='sco'
'mow' placed around 'sco' is 'MOSCOW'.
(Other definitions for moscow that I've seen before include "Bolshoi Ballet base" , "Russian capital" , "Kremlin location" , "Olympic city, 1980" , "capital city" .)