Nuclear power source unstable at core on river (7)
I believe the answer is:
'nuclear power source' is the definition.
(a nuclear reactor)
'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).
'atcore' is an anagram of 'reacto'.
(Other definitions for reactor that I've seen before include "Producer of nuclear energy" , "Nuclear power plant" , "See 3 Down" , "Nuclear energy generator" , "nuclear vessel" .)