Medical practitioner dwelling by old track (9)
I believe the answer is:
homeopath
'medical practitioner' is the definition.
'dwelling by old track' is the wordplay.
'dwelling by' becomes 'home' ('home' can be a synonym of 'dwelling'. I am not sure about the 'by' bit.**).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'track' becomes 'path' ('path' can be a synonym of 'track').
'home'+'o'+'path'='HOMEOPATH'
(Other definitions for homeopath that I've seen before include "he treats you, naturally" , "Alternative practitioner" , "therapist" , "Health practitioner" , "one offers treatment" .)