Agile doctor in African river (6)
I believe the answer is:
nimble
'agile' is the definition.
(synonyms)
'doctor in african river' is the wordplay.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'in' indicates putting letters inside.
'african river' becomes 'nile' (I've seen this before).
'mb' put within 'nile' is 'NIMBLE'.
(Other definitions for nimble that I've seen before include "Quick, agile" , "Fast; light in movement" , "Agile - alert" , "Quick on the feet" , "Swift, active" .)