Being grim, a son needs reforming (8)
I believe the answer is:
organism
'being' is the definition.
(synonyms)
'grim a son needs reforming' is the wordplay.
'needs reforming' indicates an anagram (I've seen 'needing reform' mean this).
'grim'+'a'+'son'='grimason'
'grimason' is an anagram of 'ORGANISM'.
(Other definitions for organism that I've seen before include "Living thing -- man's giro (anag)" , "Any living entity" , "Living animal or vegetable" , "Individual life form" , "Something alive" .)