Crossword Genius

Twiddle one's thumbs when ending letter (3)

Ross

I believe the answer is:

ess

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

'letter' is the definition.
(the letter S)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ess that I've seen before include "Curvaceous letter" , "top secret" , "Letter from Mid-West" , "Nineteenth letter" , "Alphabet letter" .)

I've seen this clue in The Independent.
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