Flaccid alien clings to a rock (6)

I believe the answer is:

limpet

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'clings to a rock' is the definition.
I can't judge whether this defines the answer.

'flaccid alien' is the wordplay.
'flaccid' becomes 'limp' (synonyms).
'alien' becomes 'et' (extra terrestrial).
'limp'+'et'='LIMPET'

Can you help me to learn more?

(Other definitions for limpet that I've seen before include "Marine creature clinging to a rock" , "Mollusc that clings to rocks" , "Proverbially clinging shellfish" , "Clinging mollusc" , "Proverbially clingy mollusc" .)

I've seen this clue in The Mirror.

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