Fission device unstable at core on river (7)
I believe the answer is:
reactor
'fission device' is the definition.
(I've seen this in another clue)
'unstable at core on river' is the wordplay.
'unstable' indicates an anagram.
'on' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation for river).
'at'+'core'='atcore'
'atcore' is an anagram of 'reacto'.
'reacto'+'r'='REACTOR'
(Other definitions for reactor that I've seen before include "Nuclear power station core" , "Nuclear apparatus for producing energy" , "Nuclear power source" , "device that's energy-producing" , "nuclear vessel" .)