Quick: doctor seen in river! (6)
I believe the answer is:
nimble
'quick' is the definition.
(I know that quick can be written as nimble)
'doctor seen in river' is the wordplay.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'seen in' is an insertion indicator.
'river' becomes 'nile' (Nile River is an example).
'mb' placed within 'nile' is 'NIMBLE'.
(Other definitions for nimble that I've seen before include "Light and quick in movement" , "Quick on the feet" , "Quick, agile" , "Moving quickly and lightly" , "'Jack be ......, Jack be quick' (6)." .)