Sea mud concealed mythical creature (6)
I believe the answer is:
medusa
'mythical creature' is the definition.
(Medusa is an example)
'sea mud concealed' is the wordplay.
'concealed' indicates anagramming the letters.
'sea'+'mud'='seamud'
'seamud' anagrammed gives 'MEDUSA'.
(Other definitions for medusa that I've seen before include "Form of jellyfish" , "Amused (anag)" , "One of three sisters with snakes for hair" , "Amused by the mythical gorgon?" , "all her visitors got stoned" .)