Extraordinary detail under roof of Chinese fortress (7)
I believe the answer is:
citadel
'fortress' is the definition.
(citadel can mean a fortified city)
'extraordinary detail under roof of chinese' is the wordplay.
'extraordinary' is an anagram indicator.
'under' means one lot of letters go next to another (below, in a down clue).
'roof of' says to take the initial letters (in a down clue, the top letter).
The initial letter of 'chinese' is 'c'.
'detail' is an anagram of 'itadel'.
'itadel' put after 'c' is 'CITADEL'.
(Other definitions for citadel that I've seen before include "Bastion" , "Keep - castle" , "place of refuge" , "Fortress -- dialect (anag)" , "Fortress overlooking a city" .)