Felt frequently unwell, thrashing about (8)

I believe the answer is:

flailing

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

'felt frequently unwell' is the wordplay.
'frequently' means one should take alternating letters (letters selected at a regular frequency).
'unwell' becomes 'ailing' (synonyms).
The alternate letters of 'felt' are 'fl'.
'fl'+'ailing'='FLAILING'

(Another definition for flailing that I've seen is " floundering".)

I've seen this clue in The Telegraph.

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