In jazz, briefly, I form a trio (5)
I believe the answer is:
triad
'a trio' is the definition.
('triad' can be a synonym of 'trio')
'in jazz briefly i form' is the wordplay.
'in' indicates putting letters inside.
'jazz' becomes 'trad' (trad jazz is a type of jazz).
'briefly' means to remove the last letter.
'form' becomes 'd' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'trad' with its final letter taken off is 'tra'.
'tra' enclosing 'i' is 'tria'.
'tria'+'d'='TRIAD'
Can you help me to learn more?
(Other definitions for triad that I've seen before include "Group or set of three" , "Criminals" , "three making a sound?" , "Set of three; chord" , "trio" .)