Scope -- a doctor needs it (5)
I believe the answer is:
ambit
'scope' is the definition.
('ambit' can be a synonym of 'scope')
'a doctor needs it' is the wordplay.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'needs' means one lot of letters go next to another.
'a'+'mb'+'it'='AMBIT'
(Other definitions for ambit that I've seen before include "Scope or confines" , "Scope or circuit" , "Scope, extent, or bounds of something" , "Circuit, compass" , "Compass, scope" .)