Some drip gets sloshed: guaranteed! (8)
I believe the answer is:
promised
'guaranteed' is the definition.
(thesaurus)
'some drip gets sloshed' is the wordplay.
'gets sloshed' indicates anagramming the letters.
'some'+'drip'='somedrip'
'somedrip' is an anagram of 'PROMISED'.
(Other definitions for promised that I've seen before include "swore" , "undertaken" , "Undertook (to do something)" , "Gave one's word, pledged" , "Made a commitment to do" .)