Girl John tipped over on lake (7)

I believe the answer is:

valerie

'girl' is the definition.

'john tipped over on lake' is the wordplay.
'john' becomes 'lav' (both can mean a toilet).
'tipped over' shows that the letters should be reversed in order.
'on' means one lot of letters go next to another.
'lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes').
'lav' back-to-front is 'val'.
'val'+'erie'='VALERIE'

(Another definition for valerie that I've seen is " Girl".)

I've seen this clue in The Times.

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