Pay attention when heartlessly heckled (4)
I believe the answer is:
heed
'pay attention' is the definition.
(thesaurus)
'heartlessly heckled' is the wordplay.
'heartlessly' suggests removing the centre.
'heckled' with its middle removed is 'HEED'.
'when' acts as a link.
(Other definitions for heed that I've seen before include "Pay close attention to" , "Attend to, mind" , "Observe" , "Care, attention (paid)" , "Take note of" .)