In the music trade, it’s jazz (4)

I believe the answer is:

trad

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'it's jazz' is the definition.
(trad jazz is a type of jazz)

I cannot understand how the remainder of the clue works.

(Other definitions for trad that I've seen before include "Classical (abbr)" , "Conventional (jazz)" , "music" , "(Of jazz) traditional" , "Type of jazz or folk" .)

I've seen this clue in the Evening Standard.

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