Cut up heart that fills pastry (9)
I believe the answer is:
mincemeat
'that fills pastry' is the definition.
'cut up heart' is the wordplay.
'cut up' becomes 'mince' (I've seen this before).
'heart' becomes 'meat' ('meat' can be a synonym of 'heart').
'mince'+'meat'='MINCEMEAT'
(Other definitions for mincemeat that I've seen before include "pie-fillers" , "Spiced mixture of raisins, candied peel, apples etc" , "What did one make of the defeated" , "made of one that is thoroughly beaten?" , "Christmas pie filling" .)