What's strong Italian wine in box, mostly new (7)

I believe the answer is:

bastion

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'what's strong italian' is the definition.
I can't tell whether this defines the answer.

'wine in box mostly new' is the wordplay.
'wine' becomes 'asti' (Italian wine).
'in' means one lot of letters goes inside another.
'mostly' means to remove the last letter (most but not all of the word).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'box' with its last letter removed is 'bo'.
'asti' placed into 'bo' is 'bastio'.
'bastio'+'n'='BASTION'

Can you help me to learn more?

(Other definitions for bastion that I've seen before include "Fortified place near hostile territory" , "'Stronghold, citadel (7)'" , "part of stronghold" , "Obtains (anag.)" , "Stronghold, fortification" .)

I've seen this clue in The Times.

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