Active doctor diving into river (6)
I believe the answer is:
nimble
'active' is the definition.
(similar in meaning)
'doctor diving into river' is the wordplay.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'diving into' is an insertion indicator.
'river' becomes 'nile' (Nile is an example).
'mb' inserted inside 'nile' is 'NIMBLE'.
(Other definitions for nimble that I've seen before include "Fast; light in movement" , "Moving quickly and lightly" , "Quick on the feet" , "Agile, sprightly" , "'Agile, spry (6)'" .)