Building entrance unknown for a long time (9)
I believe the answer is:
centenary
'a long time' is the definition.
(period of a hundred years)
'building entrance unknown' is the wordplay.
'building' is an anagram indicator.
'unknown' becomes 'y' (algebra).
'entrance' with letters rearranged gives 'centenar'.
'centenar'+'y'='CENTENARY'
'for' is the link.
(Other definitions for centenary that I've seen before include "special occasion" , "significant anniversary" , "100th anniversary" , "One hundredth anniversary" , "celebration" .)