Crossword Genius

Building entrance unknown for a long time (9)

Ross

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" .)

I've seen this clue in The Guardian.
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