Finely chop heart for pie filling (9)
I believe the answer is:
mincemeat
'pie filling' is the definition.
(potential filling for a pie)
'finely chop heart' is the wordplay.
'finely chop' becomes 'mince'.
'heart' becomes 'meat' (synonyms).
'mince'+'meat'='MINCEMEAT'
'for' is the link.
(Other definitions for mincemeat that I've seen before include "Christmas pie filler" , "What did one make of the defeated" , "made of one that is thoroughly beaten?" , "pie-fillers" , "Spiced mixture of raisins, candied peel, apples etc" .)