Crossword Genius

One London building, or two? (5,6)

Ross

I think it could be either:

tower bridge

whole number

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'two?' is the definition.
(two is a kind of whole number)

'one london building' is the wordplay.
'one' becomes 'whole' (synonyms).
'london building' becomes 'number' (I am not sure about this - if you are sure you should believe this answer much more).
'whole'+'number'='WHOLE NUMBER'

'or' is the link.

Can you help me to learn more?

I've seen this clue in The Mirror.
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