Crossword Genius

Where one sees bathing cap — Ondine's topper? (4)

Ross

I believe the answer is:

lido

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

'where one sees bathing' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'cap ondine's topper?' is the wordplay.
'cap' becomes 'lid' (eg the lid of a bottle).
'ondine's topper?' becomes 'o' (I can't justify this - if you can you should give a lot more credence to this answer).
'lid'+'o'='LIDO'

Can you help me to learn more?

(Other definitions for lido that I've seen before include "---- di Jesolo, Italian Adriatic resort" , "Idol (anag.)" , "Italian bathing place" , "Outdoor swimming pool" , "Swimming bath" .)

I've seen this clue in The Independent.
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