Spelling nigh impossible, old flame claimed (6)

I believe the answer is:

hexing

'spelling' is the definition.
(I've seen this in another clue)

'nigh impossible old flame claimed' is the wordplay.
'impossible' is an anagram indicator (impossible ordering of the letters).
'old flame' becomes 'ex' (an ex-lover).
'claimed' indicates putting letters inside.
'nigh' anagrammed gives 'hing'.
'hing' enclosing 'ex' is 'HEXING'.

I've seen this clue in The Telegraph.

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