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' means one lot of letters goes inside another.
'jazz' becomes 'trad' (trad jazz is a type of jazz).
'briefly' means to remove the last letter.
'form' becomes 'd' (I can't justify this - if you can you should believe this answer much more).
'trad' with its last 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" .)