One faces illness seldom (2,4)

I believe the answer is:

if ever

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

'faces illness seldom' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'one' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
The remaining letters 'fever' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Guardian.

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