Boat’s propeller observed initially beside a river (3)
I believe the answer is:
oar
'boat's propeller' is the definition.
(an oar helps propel a boat)
'observed initially beside a river' is the wordplay.
'initially' indicates taking the first letters.
'beside' says to put letters next to each other (I've seen this in other clues).
'river' becomes 'r' (abbreviation for river).
The initial letter of 'observed' is 'o'.
'o'+'a'+'r'='OAR'
(Other definitions for oar that I've seen before include "Scull" , "It ends in blade" , "Propelling blade" , "Boat propellant" , "Sportsman" .)