Starts to understand when sewers are ready (7,2)

I believe the answer is:

cottons on

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'starts to understand' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'sewers are ready' is the wordplay.
'sewers' becomes 'cottons' (I can't explain this - if you can you should believe this answer much more).
'are ready' becomes 'on' (if a device is on it is ready for use).
'cottons'+'on'='COTTONS ON'

'when' is the link.

Can you help me to learn more?

(Another definition for cottons on that I've seen is " Understands".)

I've seen this clue in the Sydney Morning Herald.

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