Jazz band -- old doctor in company (5)
I believe the answer is:
combo
'jazz band' is the definition.
('combo' can be a synonym of 'jazz band')
'old doctor in company' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'in' indicates putting letters inside.
'company' becomes 'co' (abbreviation).
'o'+'mb'='omb'
'omb' placed into 'co' is 'COMBO'.
(Other definitions for combo that I've seen before include "Music group" , "Small pop group" , "Small (jazz or rock) band" , "musicians" , "Small jazz group" .)