Horrid year after Stan went off (5)
I believe the answer is:
nasty
'horrid' is the definition.
(thesaurus)
'year after stan went off' is the wordplay.
'year' becomes 'y' (**).
'after' says to put letters next to each other.
'went off' is an anagram indicator.
'stan' is an anagram of 'nast'.
'y' put after 'nast' is 'NASTY'.
(Other definitions for nasty that I've seen before include "Evil" , "Vile" , "Awful" , "Repulsive" , "Horrible" .)