Firm is business without any piece of legislation (7)
I believe the answer is:
compact
'firm' is the definition.
('compact' can be similar in meaning to 'firm')
'business without any piece of legislation' is the wordplay.
'business' becomes 'company' (I have seen 'Commercial business ' mean 'company' so perhaps 'business' could also mean 'company').
'without' suggests deleting specific letters.
'piece of legislation' becomes 'act'.
'company' with 'any' taken away is 'comp'.
'comp'+'act'='COMPACT'
'is' is the link.
(Other definitions for compact that I've seen before include "Condense" , "The powder-box is closely packed" , "Mutual agreement" , "Dense; powder case" , "Close - agreement - make-up case" .)