Thus the Italian's entered tower (4)
I believe the answer is:
silo
'tower' is the definition.
(silo is a kind of tower)
'thus the italian's entered' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'the italian' becomes 'il' (the in Italian).
'entered' is an insertion indicator.
'so' enclosing 'il' is 'SILO'.
(Other definitions for silo that I've seen before include "Missile launcher; farm store" , "Storage unit for animal feed" , "Grain storage area" , "Launch site" , "Storage tower for oils" .)