Shed next to empty lighthouse, a grand edifice (6)
I believe the answer is:
castle
'edifice' is the definition.
(I know that castle is a type of fortification)
'shed next to empty lighthouse' is the wordplay.
'shed' becomes 'cast' (shed can mean to discard or cast off).
'next to' is a charade indicator (letters next to each other).
'empty' says to hollow out the word (remove centre letters).
'lighthouse' with its middle removed is 'le'.
'cast'+'le'='CASTLE'
'a grand' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for castle that I've seen before include "Man on board" , "Let sac be a chess piece" , "Stronghold - chessman" , "splendid house" , "E.g. Harlech, Beaumaris, Windsor" .)