Christmas book lacking heart (4)
I believe the answer is:
noel
'christmas' is the definition.
(synonyms)
'book lacking heart' is the wordplay.
'book' becomes 'novel' (novel is a kind of book).
'lacking heart' suggests removing the centre.
'novel' with its centre removed is 'NOEL'.
(Other definitions for noel that I've seen before include "Christmastime" , "Festivity" , "Coward for one" , "A lone French Christmas" , "Sir - - Coward" .)