Colonial home completely full of books (mostly kept out of sight) (3,4)
I believe the answer is:
ant hill
'colonial home' is the definition.
'completely full of books mostly kept out of sight' is the wordplay.
'completely' becomes 'all' ('all' can be a synonym of 'completely'**).
'full of' indicates putting letters inside.
'books' becomes 'nt' (New Testament).
'mostly' means to remove the last letter.
'kept out of sight' becomes 'hid' (I've seen this before).
'hid' with its last letter removed is 'hi'.
'nt'+'hi'='nthi'
'all' placed around 'nthi' is 'ANT-HILL'.
(Other definitions for ant hill that I've seen before include "colony of insects" , "Home for social workers?" , "Termite mound" , "soldiers' home?" , "All thin (anag.)" .)