Wrongdoer gathering demise in advance (8)
I believe the answer is:
offender
'wrongdoer' is the definition.
(I know that wrongdoer can be written as offender)
'gathering demise in advance' is the wordplay.
'gathering' indicates putting letters inside (gather can mean to acquire or bring in).
'demise' becomes 'end' (I've seen this in other clues).
'in advance' becomes 'offer' (I am not sure about the 'in' bit.).
'end' placed within 'offer' is 'OFFENDER'.
Can you help me to learn more?
(Other definitions for offender that I've seen before include "Transgressor, lawbreaker" , "causing resentment?" , "Delinquent" , "miscreant" , "One breaking rule" .)