Clergyman forgetting first offence (5)
I believe the answer is:
arson
'offence' is the definition.
(I've seen this before)
'clergyman forgetting first' is the wordplay.
'clergyman' becomes 'parson' (parson is a kind of clergyman).
'forgetting first' means to remove the first letter.
'parson' with its first letter taken away is 'ARSON'.
(Other definitions for arson that I've seen before include "Incendiary crime" , "The act of firing" , "Fire-lighting crime" , "Roans (anag)" , "Crime of fire-setting (5)" .)