Company doctor introducing old jazz group (5)
I believe the answer is:
combo
'jazz group' is the definition.
(synonyms)
'company doctor introducing old' is the wordplay.
'company' becomes 'co' (abbreviation).
'doctor' becomes 'mb' (abbreviation for Bachelor of Medicine).
'introducing' indicates taking the first letters.
The first letter of 'old' is 'o'.
'co'+'mb'+'o'='COMBO'
(Other definitions for combo that I've seen before include "musicians" , "Small pop group" , "group of players" , "Music group" , "Small jazz or dance band" .)