Sort of glass construction in the sea (4)
I believe the answer is:
pier
I believe this clue is a double definition.
'sort of glass' is the first definition.
(pier glass is a type of glass)
'construction in the sea' is the second definition.
(structure by a beach)
(Other definitions for pier that I've seen before include "Support member" , "Supporting structure" , "One jutting into sea" , "Mole, wall between windows" , "Surviving antiquity" .)