Put new mortar in after salvo evacuated offensive detail? (4,5)

I believe the answer is:

sore point

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'put new mortar' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'in after salvo evacuated offensive detail?' is the wordplay.
'in after salvo' becomes 'so' (I can't explain this - if you can you should believe this answer much more).
'evacuated' means to remove the middle letters.
'offensive' becomes 'rude'.
'detail?' becomes 'point' ('point' can be a synonym of 'detail').
'rude' with its centre removed is 're'.
'so'+'re'+'point'='SORE POINT'

Can you help me to learn more?

(Other definitions for sore point that I've seen before include "grievance" , "Annoying issue" , "Touchy matter" .)

I've seen this clue in The Telegraph.

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