Heartlessly cheer defeat (4)
I believe the answer is:
back
'defeat' is the definition.
(thesaurus)
'heartlessly cheer' is the wordplay.
'heartlessly' suggests removing the centre.
'cheer' becomes 'barrack' (synonyms).
'barrack' with its centre removed is 'BACK'.
(Other definitions for back that I've seen before include "Place where secret deals are done" , "Stern" , "player in line of defence?" , "Help!" , "bet" .)