Theatre is nothing to heartless devil (5)
I believe the answer is:
odeon
'theatre' is the definition.
(in ancient Greece)
'to heartless devil' is the wordplay.
'to' becomes 'o' (I can't explain this - if you can you should believe this answer much more).
'heartless' suggests removing the centre.
'devil' becomes 'demon' (synonyms).
'demon' with its centre removed is 'deon'.
'o'+'deon'='ODEON'
'is nothing' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for odeon that I've seen before include "Ancient theatre - modern cinema" , "Hal" , "Ancient Greek building for music" , "Classical Greek concert hall" , "Performances here no longer" .)