Unpleasant letters often asterisked among unknown characters (5)

I believe the answer is:

yucky

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'unpleasant' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?

I cannot understand how the rest of the clue works.

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(Other definitions for yucky that I've seen before include "Revolting" , "Messy, disgusting" .)

I've seen this clue in The Independent.

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