Disobey mad gran's sister, leaving Italy (10)
I believe the answer is:
transgress
'disobey' is the definition.
(thesaurus)
'mad gran's sister leaving italy' is the wordplay.
'mad' is an anagram indicator.
'leaving' is a deletion indicator.
'italy' becomes 'i'.
'grans'+'sister'='granssister'
'granssister' with 'i' removed is 'granssster'.
'granssster' is an anagram of 'TRANSGRESS'.
(Other definitions for transgress that I've seen before include "Cause offence" , "misbehave" , "commit offence" , "Overstep, exceed" , "Break (limit, law)" .)