...and tour decrepit domed building (7)
I believe the answer is:
rotunda
'domed building' is the definition.
(I've seen this before)
'and tour decrepit' is the wordplay.
'decrepit' indicates an anagram.
'and'+'tour'='andtour'
'andtour' anagrammed gives 'ROTUNDA'.
(Other definitions for rotunda that I've seen before include "domed hall" , "Round domed room" , "Italian Pantheon, for instance" , "Circular building with a dome" , "Do a turn (anag) -- circular domed building" .)