Book for a boy about five (5)
I believe the answer is:
novel
'book' is the definition.
(I know that novel is a type of book)
'a boy about five' is the wordplay.
'a boy' becomes 'noel' (I've seen this before).
'about' means one lot of letters goes inside another.
'five' becomes 'V' (Roman numeral).
'noel' enclosing 'v' is 'NOVEL'.
'for' is the link.
(Other definitions for novel that I've seen before include "Romance - new" , "Unusual; book" , "Kim, eg" , "Scoop, say" , "New piece of fiction?" .)