Crossword Genius

Wearing when fooling (6,2)

Ross

I believe the answer is:

having on

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

'wearing' is the definition.
('have on' can be a synonym of 'wear')

'when fooling' is the wordplay.
I cannot really see how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is located in the answer.
The remaining letters 'having' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in the Evening Standard.
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