Twiddle one's thumbs when ending letter (3)
I believe the answer is:
ess
'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" .)