Eel upset mayor (5)
I believe the answer is:
moray
'eel' is the definition.
(I know that moray is a type of eel)
'upset mayor' is the wordplay.
'upset' indicates an anagram.
'mayor' with letters rearranged gives 'MORAY'.
(Other definitions for moray that I've seen before include "Predator eel" , "Firth" , "is fishy" , "seen snaking near the coast?" , "One fish" .)