Golfing assistants ignoring European’s coarse yarn (6)

I believe the answer is:

caddis

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'yarn' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the rest of the clue.

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(Other definitions for caddis that I've seen before include "Aquatic larva of fly" , "Immature swimmer" , "Mothlike insect's larva" , "worm" , "Fly of ponds and streams that camouflages itself" .)

I've seen this clue in The Times.

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