A big book hotel's kept in reception (2,4)
I believe the answer is:
at home
'reception' is the definition.
(at home is a kind of reception)
'a big book hotel's kept' is the wordplay.
'big book' becomes 'tome' (term for a large or heavy book).
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'kept' is an insertion indicator.
'tome' going around 'h' is 'thome'.
'a'+'thome'='AT-HOME'
'in' acts as a link.
(Other definitions for at home that I've seen before include "In (for a party?)" , "celebration" , "Comfortable - in one's own country" , "ready to receive visitors" , "Reception in one's own abode" .)