Crossword Genius

Problem with fool replacing oxygen in space (6)

Ross

I believe the answer is:

hassle

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

'problem' is the definition.
(I've seen this before)

'fool replacing oxygen in space' is the wordplay.
I cannot quite see how this works, but
'fool' could be 'ass' (both can mean a stupid person) and 'ass' is located in the answer.
This explanation may well be incorrect...

'with' acts as a link.

Can you help me to learn more?

(Other definitions for hassle that I've seen before include "Harass; pester" , "Informally annoy or bother" , "Harass - grief" , "Annoy persistently" , "Annoy continually" .)

I've seen this clue in The Times.
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