Rower initially on a river (3)
I believe the answer is:
oar
'rower' is the definition.
(I've seen this in another clue)
'initially on a river' is the wordplay.
'initially' indicates taking the first letters.
'river' becomes 'r' (abbreviation).
The first letter of 'on' is 'o'.
'o'+'a'+'r'='OAR'
(Other definitions for oar that I've seen before include "Implement to propel a boat" , "Pusher" , "Rower's implement" , "Boat propellant" , "One of an eight" .)