Felt frequently unwell, thrashing about (8)
I believe the answer is:
'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'.
(Another definition for flailing that I've seen is "floundering".)