Swamp critter native to volcanic rock (4)

I believe the answer is:

croc

I'm a little stuck... Click here to teach me more about this clue!

'swamp critter' is the definition.
(I've seen this in another clue)

'native to volcanic rock' is the wordplay.
'CROC' is hidden within the letters.
But, I am unsure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for croc that I've seen before include "Snapper" , "African beast; sounds like piece of pot" , "Short reptile" , "Predatory reptile (informal)" , "Small animal" .)

I've seen this clue in The Times.

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