It's plain the Italian's in usually (10)
I believe the answer is:
ordinarily
'usually' is the definition.
(synonyms)
'it's plain the italian's in' is the wordplay.
'it's plain' becomes 'ordinary' (I've seen this before).
'the italian' becomes 'il' ('the' in Italian).
'in' means one lot of letters goes inside another.
'ordinary' enclosing 'il' is 'ORDINARILY'.
(Other definitions for ordinarily that I've seen before include "In the usual run of things" , "As a rule" , "following the custom" .)