Fussy creature's heartless (4)
I believe the answer is:
anal
'fussy' is the definition.
(anal can mean overly fussy or orderly)
'creature's heartless' is the wordplay.
'creature' becomes 'animal' (synonyms).
'heartless' means to remove the middle letters.
'animal' with its centre removed is 'ANAL'.
(Other definitions for anal that I've seen before include "Excessively orderly and fussy" , "Obsessive" .)