Poison for which the cure includes a warm heart! (6)
I believe the answer is:
curare
'poison' is the definition.
(curare is a poisonous plant)
'the cure includes a warm heart' is the wordplay.
'includes' is an insertion indicator.
'heart' says to take the centre.
The centre of 'warm' is 'ar'.
'cure' placed around 'ar' is 'CURARE'.
'for which' is the link.
(Other definitions for curare that I've seen before include "Poison applied to darts" , "Paralysing plant poison" , "Poison used on darts" , "Poison put on arrows" , "Arrow poison" .)