Crossword Genius

Cotton on tick (5-4,6)

Ross

I believe the answer is:

penny-drop moment

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

'cotton' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'tick' is the wordplay.
I cannot really see how this works, but
'tick' could be 'moment' (tick can mean a short moment) and 'moment' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

'on' is the link.

Can you help me to learn more?

I've seen this clue in The Telegraph.
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