Old doctor entering company group (5)
I believe the answer is:
combo
'group' is the definition.
The answer is an example.
'old doctor entering company' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'entering' means one lot of letters goes inside another.
'company' becomes 'co' (abbreviation).
'o'+'mb'='omb'
'omb' put inside 'co' is 'COMBO'.
(Other definitions for combo that I've seen before include "Small pop group" , "group of players" , "Music group" , "musicians" , "Small jazz or dance band" .)