Marched round building (4)

I believe the answer is:

arch

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

'building' is the definition.

'marched round' is the wordplay.
'ARCH' is hidden in amongst the letters.
However, I'm not sure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for arch that I've seen before include "This enemy is major" , "Bridge span" , "Cunning; span" , "(archaic)" , "Curved aperture" .)

I've seen this clue in The Mirror.

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