A number of penitents (3)

I believe the answer is:

ten

'a number' is the definition.
(ten is a number)

'of penitents' is the wordplay.
'of' indicates a hidden word.
'TEN' is hidden within 'penitents'.

(Other definitions for ten that I've seen before include "Five times two" , "No of sides of a decagon" , "time" , "Base of decimal system" , "Half a score" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.