Crossword Genius

Son in covert reconnaissance unit displays nerve (4)

Ross

I believe the answer is:

sass

'nerve' is the definition.
(both can mean cheekiness)

'son in covert reconnaissance unit' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'in' indicates putting letters inside.
'covert reconnaissance unit' becomes 'SAS' (Special Air Service of the British Army).
's' put within 'sas' is 'SASS'.

'displays' is the link.

(Another definition for sass that I've seen is " Cheek".)

I've seen this clue in The Times.
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