Sea nymph entangled in reed (6)
I believe the answer is:
nereid
'sea nymph' is the definition.
('Nereid' can be a synonym of 'sea nymph')
'entangled in reed' is the wordplay.
'entangled' is an anagram indicator.
'in'+'reed'='inreed'
'inreed' is an anagram of 'NEREID'.
(Other definitions for nereid that I've seen before include "Bristle worm" , "End ire (anag) - sea nymph" , "Sea nymph in greek mythology (6)" , "Sea-nymph" , "Mythological sea nymph" .)