Firm about an offence where bets are placed (6)
I believe the answer is:
casino
'where bets are placed' is the definition.
(you can bet in a casino)
'firm about an offence' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'about' indicates putting letters inside.
'an' becomes 'a'.
'offence' becomes 'sin' (a sin is an offence or transgression).
'a'+'sin'='asin'
'co' placed around 'asin' is 'CASINO'.
(Other definitions for casino that I've seen before include "enterprise?" , "--- Royale (Fleming)" , "Gambling establishment" , "Imitate" , "Place for better" .)