Crossword Genius

All of them may be crawled on (5)

Ross

I believe the answer is:

fours

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

'them may be crawled on' is the definition.
I can't judge whether this defines the answer.

'all of' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is found in the answer.
The remaining letters 'furs' 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 Evening Standard.
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