In the music trade, it's jazz (4)
I believe the answer is:
trad
'jazz' is the definition.
(trad jazz is a type of jazz)
'in the music trade' is the wordplay.
'in' indicates putting letters inside.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'music' becomes 'd'.
'trade' becomes 'ra' (I can't explain this - if you can you should believe this answer much more).
't'+'d'='td'
'td' enclosing 'ra' is 'TRAD'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for trad that I've seen before include "Classical (abbr)" , "Conventional (jazz)" , "music" , "(Of jazz) traditional" , "Type of jazz or folk" .)