Greek unit guards Yankee breakwater (6)
I believe the answer is:
groyne
'breakwater' is the definition.
('groyne' can be a synonym of 'breakwater')
'greek unit guards yankee' is the wordplay.
'greek' becomes 'gr'.
'unit' becomes 'one' ('one' can be a synonym of 'unit'**).
'guards' indicates putting letters inside.
'yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'one' going around 'y' is 'oyne'.
'gr'+'oyne'='GROYNE'
(Other definitions for groyne that I've seen before include "Structure built to control coastal erosion" , "Sea wall" , "Anti-erosion breakwater" , "A bulwark" , "Beach-protecting wall" .)