Invalid said to be a fearsome swimmer (4)

I believe the answer is:

croc

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'fearsome swimmer' is the definition.
(dangerous swimming animal)

I don't understand how the rest of the clue works.

(Other definitions for croc that I've seen before include "Short reptile" , "Large, bony-plated reptile (abbr)" , "Fierce reptile" , "Pot tailless" , "creature heard to be a vicious one" .)

I've seen this clue in The Times.

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