Nasty bit of residue on the bar tidied up (9)
I believe the answer is:
abhorrent
'nasty' is the definition.
(thesaurus)
'bit of residue on the bar tidied up' is the wordplay.
'bit of' indicates taking the first letters.
'tidied up' is an anagram indicator.
The initial letter of 'residue' is 'r'.
'r'+'on'+'the'+'bar'='ronthebar'
'ronthebar' anagrammed gives 'ABHORRENT'.
(Other definitions for abhorrent that I've seen before include "Rob in Tehran - that's disgusting" , "execrable" , "Hated" , "Repugnant, offensive" , "Inspiring disgust and loathing" .)