One of many, on the rocks, needing something to cling to (6)
I believe the answer is:
limpet
'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.
Can you help me to learn more?
(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" .)