Clergyman dismissing his first offence (5)
I believe the answer is:
arson
'offence' is the definition.
(crime of illegally starting a fire)
'clergyman dismissing his first' is the wordplay.
'clergyman' becomes 'parson' (parson is a kind of clergyman).
'dismissing his first' means to remove the first letter.
'parson' with its initial letter taken off is 'ARSON'.
(Other definitions for arson that I've seen before include "Criminal burning of property" , "burning problem" , "Pyromaniac's crime" , "Fire-lighting crime" , "Criminal pyromania" .)