Combination of notes, one used in a form of jazz (5)
I believe the answer is:
triad
'combination of notes' is the definition.
The definition and answer can be both groups as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'one used in a form of jazz' is the wordplay.
'one' becomes 'i' (Roman numeral).
'used in' indicates putting letters inside.
'a form of jazz' becomes 'trad' (I've seen this before).
'i' placed within 'trad' is 'TRIAD'.
Can you help me to learn more?
(Other definitions for triad that I've seen before include "Union of three" , "Could be major" , "Group or set of three" , "Criminals" , "Up to date" .)