Damon's are said to be old (5)

I believe the answer is:

hills

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'old' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for hills that I've seen before include "See 7" , "High terrain" , "Elevations of land" , "Heights; tors" , "Mounts" .)

I've seen this clue in the Evening Standard.

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