Criminal purpose in proposal (8)
I believe the answer is:
'criminal' is the definition.
(I know that criminal can be written as offender)
'purpose in proposal' is the wordplay.
'purpose' becomes 'end' (both can mean a goal).
'in' is an insertion indicator.
'proposal' becomes 'offer' (an offer is a proposal or suggestion).
'end' placed into 'offer' is 'OFFENDER'.
(Other definitions for offender that I've seen before include "Culprit" , "Delinquent" , "miscreant" , "Committer of illegal act" , "guilty party" .)