Marched round the building (4)

I believe the answer is:

arch

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

'the building' is the definition.

'marched round' is the wordplay.
'ARCH' is hidden within the letters.
But, I am uncertain how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for arch that I've seen before include "Curved spanning structure" , "not to go straight?" , "Affectedly playful in curved manner" , "Roguish - chief - supporter" , "Construction with a keystone" .)

I've seen this clue in The Mirror.

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