One is accepted into jazz threesome (5)
I believe the answer is:
triad
'threesome' is the definition.
(I know that threesome can be written as triad)
'one is accepted into jazz' is the wordplay.
'one is accepted' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
'into' is an insertion indicator.
'jazz' becomes 'trad' (trad jazz is a type of jazz).
'i' placed within 'trad' is 'TRIAD'.
Can you help me to learn more?
(Other definitions for triad that I've seen before include "Set of three; chord" , "Three-note chord -- Chinese criminal gang" , "three making a sound?" , "Chinese secret society" , "Up to date" .)