Imported horse won race, mostly backed (5)

I believe the answer is:

waler

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'imported horse' is the definition.
The definition suggests a singular noun which matches the answer.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Another definition for waler that I've seen is " horse from overseas".)

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

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