27 across
Powerful
ambassador
touched
on
nuclear
annihilation
(9)
Powerful ambassador touched on nuclear annihilation (9)
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The definition is at the beginning of the clue.
How the clue works
The definition is Powerful — something mighty, Hercules-like. For the wordplay, ambassador gives you HE (His Excellency), which is placed next to ("touched on") an anagram of NUCLEAR — and annihilation is your anagram indicator there — giving you RCULEAN to follow it.