Waterloo, perhaps, has its circle line (4)

I believe the answer is:

loop

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'line' is the definition.
(I know that loop is a type of Jordan curve)

'waterloo perhaps has' is the wordplay.
'has' indicates the answer is hidden within the clue.
'LOOP' is hidden within 'waterloo perhaps'.

'its circle' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for loop that I've seen before include "I cross myself" , "Circular shape of pool" , "Branch line" , "Noose" , "Closed circuit eg as hearing aid" .)

I've seen this clue in the Evening Standard.

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