Thumb book: Sepulchre (4)
I believe the answer is:
tomb
'sepulchre' is the definition.
(I've seen this before)
'thumb book' is the wordplay.
'thumb' becomes 'tom' (fairy tale character Tom Thumb).
'book' becomes 'b' (abbreviation for book).
'tom'+'b'='TOMB'
(Other definitions for tomb that I've seen before include "Crypt or grave" , "Catacomb" , "Sepulchral monument" , "Large vault" , "Sepulchre" .)