Finished circling river in unmanned aircraft (5)
I believe the answer is:
drone
'unmanned aircraft' is the definition.
(I've seen this before)
'finished circling river' is the wordplay.
'finished' becomes 'done' (both can mean completed).
'circling' is an insertion indicator (circling can mean going around or containing).
'river' becomes 'r' (abbreviation for river).
'done' enclosing 'r' is 'DRONE'.
'in' is the link.
(Other definitions for drone that I've seen before include "Idle type" , "Talk boringly; a parasite" , "Bee, speak tediously" , "mollusc" , "He buzzes, but not with energy!" .)