Cleric is source of petty crime (6)
I believe the answer is:
parson
'cleric' is the definition.
(I've seen this in another clue)
'source of petty crime' is the wordplay.
'source of' indicates taking the first letters (the start - or source - of the word).
'crime' becomes 'arson' (crime of illegally starting a fire).
The initial letter of 'petty' is 'p'.
'p'+'arson'='PARSON'
'is' acts as a link.
(Other definitions for parson that I've seen before include "service provider" , "Aprons (anag.)" , "Rector; clergyman" , "Churchman" , "A rector or vicar" .)