Small book in recess (4)
I believe the answer is:
baby
'small' is the definition.
(thesaurus)
'book in recess' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'in' is an insertion indicator.
'recess' becomes 'bay' (bay is a kind of recess).
'b' put into 'bay' is 'BABY'.
(Other definitions for baby that I've seen before include "Honey" , "Bunting" , "New-born child" , "Small child" , "Girlfriend" .)