Reconstruct Iceland's worst fortress (7,6)
I believe the answer is:
windsor castle
'fortress' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'reconstruct iceland's worst' is the wordplay.
'reconstruct' is an anagram indicator (I've seen this in other clues).
'icelands'+'worst'='icelandsworst'
'icelandsworst' anagrammed gives 'WINDSOR CASTLE'.
Can you help me to learn more?
(Other definitions for windsor castle that I've seen before include "Royal residence" , "palace" , "royal house" , "Queen's residence" .)