Fiddle discovered underneath floor (8)
I believe the answer is:
confound
'floor' is the definition.
(thesaurus)
'fiddle discovered underneath' is the wordplay.
'fiddle' becomes 'con'.
'discovered' becomes 'found' (finding is a kind of discovering).
'underneath' is a charade indicator (letters next to each other).
'con'+'found'='CONFOUND'
(Other definitions for confound that I've seen before include "Nonplus" , "Astonish and confuse" , "Thwart - baffle" , "Surprise; defeat (plan)" , "Defeat (a plan or hope)" .)