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Pigs and horses, maybe, heading off (7)

Ross

I believe the answer is:

rotters

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'pigs' is the definition.
The answer and definition can be both people as well as being plural nouns.
Perhaps you can see a link between them that I can't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Another definition for rotters that I've seen is " Contemptible folk".)

I've seen this clue in The Times.
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