Dressing always kept by doctor (4)
I believe the answer is:
mayo
'dressing' is the definition.
(mayo is a condiment or dressing)
'always kept by doctor' is the wordplay.
'always' becomes 'ay' (archaic term for 'always').
'kept by' means one lot of letters goes inside another.
'doctor' becomes 'mo' (medical officer).
'ay' put into 'mo' is 'MAYO'.
(Other definitions for mayo that I've seen before include "Irish county (county town, Castlebar)" , "County of north-west Ireland" , "Eg, Braeburn or Cox" , "You'll find it in the Emerald Isle" , "Ir. county; salad dressing" .)