No shift involving doctor in thirty days (8)
I believe the answer is:
november
'thirty days' is the definition.
(month with 30 days)
'no shift involving doctor' is the wordplay.
'shift' becomes 'veer'.
'involving' means one lot of letters goes inside another.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'veer' placed around 'mb' is 'vember'.
'no'+'vember'='NOVEMBER'
'in' acts as a link.
(Other definitions for november that I've seen before include "Mob nerve badly in winter" , "bit of a fall" , "Ninth month in the Roman calendar" , "Move Bern in this month" , "Armistice month" .)