Defence such as has been applied to port? (13)

I believe the answer is:

fortification

I'm a little stuck... Click here to teach me more about this clue!

'defence' is the definition.
(fortification is a kind of defence)

'has been applied to port?' is the wordplay.
I cannot really understand how this works, but
'applied' could be 'on' and 'on' is found within the answer.
'to' could be 'for' and 'for' is found within the answer.
This explanation may well be incorrect...

'such as' is the link.

Can you help me to learn more?

(Other definitions for fortification that I've seen before include "Strengthening" , "Eg, a defensive wall" .)

I've seen this clue in The Times.

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