Exceptionally slow start building floor (9)
I believe the answer is:
dumbfound
'floor' is the definition.
(thesaurus)
'exceptionally slow start' is the wordplay.
'exceptionally slow' becomes 'dumb' (synonyms. I am not sure about the 'exceptionally' bit.).
'start' becomes 'found' (I've seen this before).
'dumb'+'found'='DUMBFOUND'
'building' is the link.
Can you help me to learn more?
(Other definitions for dumbfound that I've seen before include "are astonishing" , "overcome with astonishment" , "Astonish, render speechless" , "Leave lost for words" , "are astounding" .)