Eels move differently (4)
I believe the answer is:
else
'differently' is the definition.
(I've seen this in another clue)
'eels move' is the wordplay.
'move' is an anagram indicator.
'eels' anagrammed gives 'ELSE'.
(Other definitions for else that I've seen before include "Other than (what is under consideration)" , "Adverb meaning if not, otherwise" , "Besides, otherwise" , "Besides; different" , "Besides; instead" .)