Slippery swimmers from Castle Elsinore (4)
I believe the answer is:
eels
'slippery swimmers' is the definition.
(I've seen this in another clue)
'from castle elsinore' is the wordplay.
'from' says the answer is hidden in the clue.
'EELS' is hidden in the letters of 'castle elsinore'.
(Other definitions for eels that I've seen before include "Slender fish" , "Long fishes" , "Long thin fish" , "Water snakes twisting in lees" , "Slithery fish" .)