Crossword Genius

Jazz up weapon (4)

Ross

I believe the answer is:

dart

'weapon' is the definition.
(dart is a kind of weapon)

'jazz up' is the wordplay.
'jazz' becomes 'trad' (trad jazz is a type of jazz).
'up' is a reversal indicator (in down clue: letters go upwards).
'trad' reversed gives 'DART'.

(Other definitions for dart that I've seen before include "Fish < from River" , "Arrow; dash" , "Devon river - bound - race" , "Seasoned beef" , "one destined to go on board?" .)

I've seen this clue in The Guardian.
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