Knox, for instance (4)

I believe the answer is:

fort

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'knox' is the definition.
Both the answer and definition are singular nouns.
Maybe they are linked in a way I don't understand?

'for instance' is the wordplay.
'instance' becomes 't' (I can't justify this - if you can you should believe this answer much more).
'for'+'t'='FORT'

Can you help me to learn more?

(Other definitions for fort that I've seen before include "Citadel" , "Strategic position" , "& 4 Down Army post in USA" , "Defensive building" , "Military post in the old West" .)

I've seen this clue in the Evening Standard.

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