Combat central threat inside castle (6)

I believe the answer is:

resist

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'combat' is the definition.
(I've seen this before)

'central threat inside castle' is the wordplay.
'central' indicates the central letters.
'inside castle' becomes 'sist' (I can't justify this - if you can you should believe this answer much more).
The central letters of 'threat' are 're'.
're'+'sist'='RESIST'

Can you help me to learn more?

(Other definitions for resist that I've seen before include "Don't give in" , "'Withstand, fight against (6)'" , "Oppose or fight against someone" , "Oppose, put up defence" , "Refuse to comply" .)

I've seen this clue in The Irish Times.

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