Sign to receive adult books in shelter (4-2)
I believe the answer is:
lean-to
'shelter' is the definition.
(lean-to is a kind of shelter)
'sign to receive adult books' is the subsidiary indication.
'sign' becomes 'leo' (Leo is an example**).
'to receive' indicates putting letters inside.
'adult' becomes 'a'.
'books' becomes 'nt' (abbreviation for New Testament).
'a'+'nt'='ant'
'leo' enclosing 'ant' is 'leanto'.
'in' is the link.
(Other definitions for lean-to that I've seen before include "Shed built onto house" , "Penthouse" , "Building built against another" , "It's roof slants against a wall" , "Building added to another" .)