They may be jellied or else cooked (4)
I believe the answer is:
eels
'they may be jellied' is the definition.
The definition suggests an adverb but the answer is not.
'else cooked' is the wordplay.
'cooked' indicates anagramming the letters (letters cooked into a new form).
'else' is an anagram of 'EELS'.
'or' is the link.
Can you help me to learn more?
(Other definitions for eels that I've seen before include "Slithery fish" , "Slippery creatures" , "Water snakes twisting in lees" , "Odd fish found in the lees" , "Else (anag.)" .)