Chief supporter (4)

I believe the answer is:

arch

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This might be a double definition.

'chief' is the first definition.
(synonyms)

'supporter' is the second definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

Can you help me to learn more?

(Other definitions for arch that I've seen before include "Structure with a keystone" , "Curved gateway" , "Masonry construction for spanning an opening" , "Curved bony structure in the foot" , "Bridge span" .)

I've seen this clue in The Telegraph and the Universal.

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