A Christmas Carol 's a heartless book (4)
I believe the answer is:
noel
'a christmas carol 's' is the definition.
('noel' can mean a Christmas carol)
'a heartless book' is the wordplay.
'a heartless' says to hollow out the word (remove centre letters).
'book' becomes 'novel' (novel is a kind of book).
'novel' with its centre taken out is 'NOEL'.
(Other definitions for noel that I've seen before include "A lone Christmas in France" , "Coward - Christmas" , "Lone male name" , "Festivity" , "Xmas refrain" .)