Fish skin, but no head (3)
I believe the answer is:
eel
'fish' is the definition.
(eel is a kind of fish)
'skin but no head' is the wordplay.
'skin' becomes 'peel' (synonyms).
'but no head' means to remove the first letter (the first letter could be called the head).
'peel' with its first letter taken away is 'EEL'.
(Other definitions for eel that I've seen before include "Teleost fish" , "Creature like conger or moray" , "Ribbon-like fish" , "Slippery swimmer (may be jellied)" , "Slithery fish" .)