Count him among the jazz greats (5)
I believe the answer is:
basie
'count' is the definition.
(I've seen this before)
'him among the jazz greats' is the wordplay.
'him' becomes 'Si'.
'among' indicates putting letters inside (inserted letters are placed among others).
'the jazz greats' becomes 'bae' (I can't justify this - if you can you should believe this answer much more).
'si' placed within 'bae' is 'BASIE'.
Can you help me to learn more?
(Other definitions for basie that I've seen before include "Count ___ , US jazz pianist, d. 1984" , "Jazz musician" , "jazz great" , "Band leader" , "Bandleader Count" .)