Optical illusion provided for US Congressmen (7)

I believe the answer is:

capitol

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

'for us congressmen' is the definition.
I can't judge whether this defines the answer.

'optical illusion' is the wordplay.
'optical' can be anagrammed to 'CAPITOL'.
But, I am not sure how the anagram is indicated.

'provided' is the link.

Can you help me to learn more?

(Other definitions for capitol that I've seen before include "Statehouse" , "Roman temple" , "Seat of US Congress" , "US assembly building" , "US Senate and House of Representatives building" .)

I've seen this clue in The Globe and Mail.

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