Firm editor rejected rules (4)

I believe the answer is:

code

'rules' is the definition.

'firm editor rejected' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'editor' becomes 'ed' (abbreviation).
'rejected' says the letters should be written in reverse.
'ed' back-to-front is 'de'.
'co'+'de'='CODE'

(Other definitions for code that I've seen before include "conventions" , "See 21 Across" , "difficult to read" , "Playfair is one" , "Ethical standards" .)

I've seen this clue in The Guardian.

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