Boy comes to end of the big book (4)
I believe the answer is:
tome
'big book' is the definition.
(I've seen this before)
'boy comes to end of the' is the wordplay.
'boy' becomes 'tom' (**).
'comes to' means one lot of letters go next to another.
'end of' suggests the final letters.
The final letter of 'the' is 'e'.
'tom'+'e'='TOME'
(Other definitions for tome that I've seen before include "dictionary?" , "Large and scholarly book" , "Large, weighty book" , "Large scholarly volume" , "War and Peace, say" .)