What could be small Scottish swallow of old (5)

I believe the answer is:

abysm

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'old' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for abysm that I've seen before include "Deep hole" , "Hell" , "Deep cavity (arch)" , "Gorge" , "Bottomless pit" .)

I've seen this clue in The Times and The Sunday Times.

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