In jazz, briefly, I form a trio (5)
I believe the answer is:
Here is my best explanation:
'a trio' is the definition.
('triad' can be a synonym of 'trio')
'in jazz briefly i form' is the wordplay.
'in' is an insertion indicator.
'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' going around 'i' is 'tria'.
Can you help me to learn more?