Event about cats and mice run badly (12)
I believe the answer is:
circumstance
'event' is the definition.
(thesaurus)
'about cats and mice run badly' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa'**).
'and' means one lot of letters go next to another (I've seen this in other clues).
'badly' indicates anagramming the letters.
'mice'+'run'='micerun'
'cats' put after 'micerun' is 'miceruncats'.
'miceruncats' anagrammed gives 'ircumstance'.
'c'+'ircumstance'='CIRCUMSTANCE'
(Other definitions for circumstance that I've seen before include "Happening" , "Situation" , "Event - detail" , "to go with pomp" , "Occurrence - fact" .)