Spend a very long time finding way through building? (7)
I believe the answer is:
passage
'way through building?' is the definition.
'spend a very long time' is the wordplay.
'spend' becomes 'pass' ('pass' can be a synonym of 'spend').
'a very long time' becomes 'age' (I've seen this before).
'pass'+'age'='PASSAGE'
'finding' acts as a link.
(Other definitions for passage that I've seen before include "Book extract; transit" , "Section of text; journey" , "Sea or air journey" , "Short section of text" , "Channel" .)