Old still drink with doctor in charge (7)
I believe the answer is:
alembic
'old still' is the definition.
(old still - device for preparing drinks)
'drink with doctor in charge' is the wordplay.
'drink' becomes 'ale' (type of beer).
'with' means one lot of letters go next to another.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'ale'+'mb'+'ic'='ALEMBIC'
(Other definitions for alembic that I've seen before include "Old retort" , "Old distillation vessel" , "Still" .)