Minister going topless is a firing offence (5)
I believe the answer is:
arson
'a firing offence' is the definition.
(criminal offence of starting a fire)
'minister going topless' is the wordplay.
'minister' becomes 'parson' (church minister).
'going topless' means to remove the first letter.
'parson' with its initial letter taken off is 'ARSON'.
'is' acts as a link.
(Other definitions for arson that I've seen before include "Criminal activity" , "Fire-raiser's crime" , "Malicious crime" , "Sonar (anag.)" , "Felonious burning of property" .)