Rower principally operating on a river (3)
I believe the answer is:
oar
'rower' is the definition.
(I've seen this in another clue)
'principally operating on a river' is the wordplay.
'principally' suggests taking the first letters.
'on' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
The initial letter of 'operating' is 'o'.
'o'+'a'+'r'='OAR'
(Other definitions for oar that I've seen before include "Propelling blade" , "Boat propellant" , "Bladed pole" , "spoon?" , "One paddling" .)