What does the cryptic crossword clue "Nuclear power source unstable at core on river (7)" mean?

The definition is "Nuclear power source". The wordplay uses anagram, charade. anagram of ATCORE ("at core") + R (abbreviation of "river") [anagram: "unstable"] = REACTOR; definition: "Nuclear power source" The answer is Reactor.

Nuclear power source unstable at core on river (7)

Telegraph #29996 | 7 Down | 2022-05-25 flower

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