Radio-controlled aircraft finished across river (5)
I believe the answer is:
drone
'radio-controlled aircraft' is the definition.
('drone' can be a synonym of 'radio-controlled aircraft')
'finished across river' is the wordplay.
'finished' becomes 'done' ('done' can be similar in meaning to 'finished').
'across' is an insertion indicator.
'river' becomes 'r'.
'done' enclosing 'r' is 'DRONE'.
(Other definitions for drone that I've seen before include "Idler or male honey-bee" , "Buzzing sound" , "Member of Wooster's club" , "Queen's mate" , "Aeroplane" .)