Crossword Genius

Drying agent tails off excessively? (5)

Ross

I believe the answer is:

towel

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'drying agent' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for towel that I've seen before include "Drier" , "Bathroom cloth" , "Dryer" , "Piece of absorbent cloth or paper for drying" , "Bathroom item" .)

I've seen this clue in The Times.
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