Finished crossing river in aircraft (5)
I believe the answer is:
drone
'aircraft' is the definition.
(drone is a kind of aircraft)
'finished crossing river' is the wordplay.
'finished' becomes 'done' ('done' can be similar in meaning to 'finished').
'crossing' means one lot of letters goes inside another.
'river' becomes 'r'.
'done' going around 'r' is 'DRONE'.
'in' acts as a link.
(Other definitions for drone that I've seen before include "boring speaker" , "mollusc" , "Idle fellow" , "Queen's mate" , "Talk boringly; male bee" .)