Skins decapitated fish (4)
I believe the answer is:
eels
'fish' is the definition.
(eel is a type of fish)
'skins decapitated' is the wordplay.
'skins' becomes 'peels' ('peel' can be a synonym of 'skin').
'decapitated' means to remove the first letter.
'peels' with its initial letter taken away is 'EELS'.
(Other definitions for eels that I've seen before include "Jellied --, traditional dish" , "Smooth fishes" , "Slippery customers" , "Anguillae" , "Devious types" .)