Fairy tale with a little slimy thing (4)
I believe the answer is:
myth
'fairy tale' is the definition.
(thesaurus)
'with a little slimy thing' is the wordplay.
'with' indicates the answer is hidden within the clue.
'MYTH' can be found hidden inside 'little slimy thing'.
(Other definitions for myth that I've seen before include "Wildly-held but false belief or idea" , "A US president; a UK PM" , "Fanciful story" , "lie!" , "Invention" .)