Crossword Genius

One of many, on the rocks, needing something to cling to (6)

Ross

I believe the answer is:

limpet

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'one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I don't understand the rest of the clue.

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(Other definitions for limpet that I've seen before include "Mollusc adhering to rocks" , "sticker?" , "Rockpool clinger?" , "It clings to rock surfaces" , "he's not easily got rid of" .)

I've seen this clue in the Evening Standard.
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