Crossword Genius

They are kept from going astray by crooks (6,2,5)

Ross

I believe the answer is:

flocks of sheep

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

'going astray by crooks' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'they are kept from' is the wordplay.
I cannot really understand how this works, but
'from' could be 'of' (I've seen this in other clues) and 'of' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Globe and Mail.
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