Culprit placed old farthing on part of fireplace (8)
I believe the answer is:
offender
'culprit' is the definition.
(I know that culprit can be written as offender)
'placed old farthing on part of fireplace' is the wordplay.
'placed' indicates an anagram (placing the letters).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'farthing' becomes 'f' (abbreviation listed in Chambers).
'on' is a charade indicator (letters next to each other).
'part of fireplace' becomes 'fender' (a fender is part of a fireplace).
'o' put after 'f' is 'fo'.
'fo' is an anagram of 'of'.
'of'+'fender'='OFFENDER'
(Other definitions for offender that I've seen before include "causing resentment?" , "One breaking rule" , "Culprit" , "Delinquent" , "miscreant" .)