Crossword Genius

Late night hour? (3)

Ross

I believe the answer is:

ten

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

'hour?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'late night' is the wordplay.
'TEN' is hidden in amongst the letters.
But, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for ten that I've seen before include "X-WORD" , "Very young" , "Dudley Moore/Bo Derek film" , "How many" , "Figure" .)

I've seen this clue in the Evening Standard.
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