Failing to follow a doctor's initial opinion (6)
I believe the answer is:
advice
'opinion' is the definition.
(I've seen this before)
'failing to follow a doctor's initial' is the wordplay.
'failing' becomes 'vice' (I've seen this in other clues).
'to follow' means one lot of letters go next to another.
'initial' suggests taking the first letters.
The initial letter of 'doctors' is 'd'.
'a'+'d'='ad'
'vice' put after 'ad' is 'ADVICE'.
(Other definitions for advice that I've seen before include "Supposedly wise words" , "What lawyer offers" , "Opinion given about what to do" , "Supposed words of wisdom" , "Suggestion for appropriate course of action" .)