Crossword Genius

Spectre flying around that is frightening in the extreme (9)

Ross

I believe the answer is:

creepiest

'frightening in the extreme' is the definition.
(extremely creepy)

'spectre flying around that is' is the wordplay.
'flying' indicates anagramming the letters (letters fly around into new positions).
'around' indicates putting letters inside.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'spectre' anagrammed gives 'creepst'.
'creepst' placed around 'ie' is 'CREEPIEST'.

(Another definition for creepiest that I've seen is " Most unsettling".)

I've seen this clue in The Times.
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