More agile doctor in the river to run (7)
I believe the answer is:
nimbler
'more agile' is the definition.
(I've seen this before)
'doctor in the river to run' is the wordplay.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'in the' is an insertion indicator.
'river' becomes 'nile' (Nile is an example).
'to run' becomes 'r' (cricket abbreviation).
'mb' placed into 'nile' is 'nimble'.
'nimble'+'r'='NIMBLER'
(Other definitions for nimbler that I've seen before include "more alert" , "Lighter moving" , "More agile" .)