Dressing always fixed by doctor (4)
I believe the answer is:
mayo
'dressing' is the definition.
(I've seen this before)
'always fixed by doctor' is the wordplay.
'always' becomes 'ay' (archaic term for 'always').
'fixed by' indicates putting letters inside (inserted letters are fixed inside).
'doctor' becomes 'mo' (medical officer).
'ay' put into 'mo' is 'MAYO'.
(Other definitions for mayo that I've seen before include "Irish county -- salad cream" , "You'll find it in the Emerald Isle" , "County of Western Eire" , "Salad dressing (informal)" , "part of Ireland" .)