Vomit left by unknown drinker (6)
I believe the answer is:
barfly
'drinker' is the definition.
(thesaurus)
'vomit left by unknown' is the wordplay.
'vomit' becomes 'barf' (synonyms).
'left by' becomes 'l' (I've seen this in other clues).
'unknown' becomes 'y' (algebra).
'barf'+'l'+'y'='BARFLY'
(Other definitions for barfly that I've seen before include "pub regular" , "frequenter of pubs" .)