Absolutely denied when sloshed (6)
I believe the answer is:
indeed
'absolutely' is the definition.
(both can be used to agree with something)
'denied when sloshed' is the wordplay.
'when sloshed' indicates an anagram.
'denied' anagrammed gives 'INDEED'.
(Other definitions for indeed that I've seen before include "Really, without doubt" , "In fact, really" , "To be sure" , "In truth, in fact" , "Actually -- denied (anag)" .)