Crossword Genius

Rated bottom of class, feared catastrophe (7,3,5)

Ross

I believe the answer is:

thought the worst

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

'class feared catastrophe' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'rated bottom of' is the wordplay.
I cannot really understand how this works, but
'bottom' could be 'worst' (similar in meaning) and 'worst' is present in the answer.
'of' could be 'o' and 'o' is found in the answer.
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 Times.
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