Heartless devil’s discovery (4)
I believe the answer is:
find
'discovery' is the definition.
(synonyms)
'heartless devil's' is the wordplay.
'heartless' means to remove the middle letters.
'devil' becomes 'fiend' (synonyms).
'fiend' with its centre removed is 'FIND'.
(Other definitions for find that I've seen before include "Acquisition" , "Discover by chance" , "Locate or discover" , "Come across" , "Discovery" .)