A doctor got up for this chap (7)
I believe the answer is:
'this chap' is the definition.
'a doctor got up' is the wordplay.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'got up' becomes 'rose' (I've seen this before).
'for' acts as a link.
(Other definitions for ambrose that I've seen before include "-- Bierce, US author", "handle for him", "Church Father", "Curtly perhaps", "name".)