Eryngo transformed breakwater (6)
I believe the answer is:
groyne
'breakwater' is the definition.
('groyne' can be a synonym of 'breakwater')
'eryngo transformed' is the wordplay.
'transformed' indicates anagramming the letters.
'eryngo' with letters rearranged gives 'GROYNE'.
(Other definitions for groyne that I've seen before include "barrier method?" , "Seaside feature" , "Mole" , "(Usually wooden) breakwater" , "Protective wall extending into water" .)