Cold heart, fanatical? (8)
I believe the answer is:
hardcore
'fanatical?' is the definition.
(hardcore can mean fanatical or extreme)
'cold heart' is the wordplay.
'cold' becomes 'hard' (both can mean unemotional).
'heart' becomes 'core' (synonyms).
'hard'+'core'='HARDCORE'
(Other definitions for hardcore that I've seen before include "The most committed members" , "foundation course?" , "Rock music" , "dance music" , "Pornography" .)