Left out heartlessly a good deal (3)
I believe the answer is:
lot
'a good deal' is the definition.
(I know that good deal can be written as lot)
'left out heartlessly' is the wordplay.
'left' becomes 'l' (common abbreviation).
'heartlessly' means to remove the middle letters.
'out' with its centre removed is 'ot'.
'l'+'ot'='LOT'
(Other definitions for lot that I've seen before include "Chance; item for sale" , "is to be knocked down" , "Could there be many at the auction?" , "Large amount at an auction" , "The French department" .)