Following after a short time, but not feeling well (6)

I believe the answer is:

ailing

I'm a little stuck... Click here to teach me more about this clue!

'not feeling well' is the definition.
(I've seen this in another clue)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ailing that I've seen before include "Infertile" , "In poor health" , "Troubled in mind or body" , "Unwell, in poor condition" , "Suffering" .)

I've seen this clue in the Evening Standard.

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