Boy for whom big book is too much (3)
I believe the answer is:
tom
'boy' is the definition.
'big book is too much' is the wordplay.
'big book' becomes 'tome' (I've seen this before).
'is too much' means to remove the last letter (not all of the word is needed).
'tome' with its last letter taken away is 'TOM'.
'for whom' acts as a link.
(Other definitions for tom that I've seen before include "Queen's mate" , "Male beast" , "What Dick Whittington had" , "queen's consort?" , "- Lehrer, US humorist" .)