The heart to spoil an argument (6)
I believe the answer is:
marrow
'the heart' is the definition.
('marrow' can be a synonym of 'heart')
'spoil an argument' is the wordplay.
'spoil' becomes 'mar' (I've seen this before).
'an argument' becomes 'row' (I've seen this before).
'mar'+'row'='MARROW'
'to' is the link.
(Other definitions for marrow that I've seen before include "Vegetable - tissue in bone" , "essential part" , "Oversized courgette" , "Stuff found inside bones" , "Bone filler" .)