Crossword Genius

What reviewer usually offers maybe beyond which reflection less than total (8,5)

Ross

I believe the answer is:

critical angle

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

'beyond which reflection less than total' is the definition.
(angle beyond which rays are refracted)

'what reviewer usually offers maybe' is the wordplay.
I cannot really see how this works, but
'reviewer' could be 'critic' (I've seen this before) and 'critic' is present in the answer.
This may be the basis of 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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