Clean horrible stain on one's evening top (8)
I believe the answer is:
sanitise
'clean' is the definition.
('clean'->'sanitize' is in my internal thesaurus and 'ize'->'ise')
'horrible stain on one's evening top' is the wordplay.
'horrible' is an anagram indicator.
'on' says to put letters next to each other.
'one's' becomes 'is' (roman numeral**).
'top' indicates taking the first letters.
The initial letter of 'evening' is 'e'.
'stain' is an anagram of 'sanit'.
'sanit'+'is'+'e'='SANITISE'
(Other definitions for sanitise that I've seen before include "Clean and sterilize" , "block out unpleasant things" , "Make hygienic being in siesta" , "Make hygienic, sterilise" , "disinfect" .)