I think it's a book (4)
I believe the answer is:
tome
'a book' is the definition.
(a tome is a book)
'i think it's' is the wordplay.
'i think' becomes 'tom' (I can't justify this - if you can you should give a lot more credence to this answer).
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'tom'+'e'='TOME'
Can you help me to learn more?
(Other definitions for tome that I've seen before include "Large heavy book" , "Large and scholarly book" , "War and Peace, say" , "dictionary?" , "Large, weighty book" .)