Seaside structure (4)
I believe the answer is:
pier
'seaside structure' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pier that I've seen before include "sight at the seaside" , "'Dock, wharf (4)'" , "Mole, wall between windows" , "Breakwater; arch pillar" , "Wharf jutting out into water" .)