Hymn book lacking centrepiece (4)
I believe the answer is:
noel
'hymn' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'book lacking centrepiece' is the wordplay.
'book' becomes 'novel' (novel is a kind of book).
'lacking centrepiece' means to remove the middle letters.
'novel' with its centre removed is 'NOEL'.
Can you help me to learn more?
(Other definitions for noel that I've seen before include "Christmas; man's name" , "Coward for one" , "namely 4 [COWARD]" , "Christmastime" , "See 7 Across" .)