No grey rocks in beach structure (6)
I believe the answer is:
groyne
'beach structure' is the definition.
(type of barrier built on a beach)
'no grey rocks' is the wordplay.
'rocks' indicates anagramming the letters.
'no'+'grey'='nogrey'
'nogrey' anagrammed gives 'GROYNE'.
'in' is the link.
(Other definitions for groyne that I've seen before include "Anti-erosion breakwater" , "Structure built to control coastal erosion" , "Erosion-control wall" , "Barrier built on beach" , "A bulwark" .)