Scurrilous lost soul with the mare's tails in The Cloud (6)

I believe the answer is:

cirrus

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'cloud' is the definition.
(cirrus is a kind of cloud)

I don't understand the rest of the clue.

(Other definitions for cirrus that I've seen before include "High-level fair weather cloud" , "Wipsy white cloud" , "High wispy cloud" , "Wispy high-altitude cloud" , "High altitude cloud" .)

I've seen this clue in the Irish Times.

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