Firm is checking amateur window cleaner (7)
I believe the answer is:
chamois
'window cleaner' is the definition.
(I've seen this before)
'firm is checking amateur' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'checking' means one lot of letters goes inside another (check can mean to control or arrest).
'amateur' becomes 'ham' (as in ham radio).
'co'+'is'='cois'
'cois' going around 'ham' is 'CHAMOIS'.
(Other definitions for chamois that I've seen before include "European antelope" , "Soft pliable leather" , "Mountain antelope - soft leather" , "Agile hoofed animal - polishing cloth" , "Leather; goat-antelope" .)