Swift river trapping doctor (6)
I believe the answer is:
nimble
'swift' is the definition.
(I've seen this before)
'river trapping doctor' is the wordplay.
'river' becomes 'nile' (Nile is an example**).
'trapping' is an insertion indicator.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'nile' enclosing 'mb' is 'NIMBLE'.
(Other definitions for nimble that I've seen before include "Agile - alert" , "'Agile, spry (6)'" , "Agile, sprightly" , "Fast; light in movement" , "Quick on the feet" .)