Catastrophe if a rule's broken (7)
I believe the answer is:
failure
'catastrophe' is the definition.
(thesaurus)
'if a rule's broken' is the wordplay.
'broken' is an anagram indicator.
'if'+'a'+'rule'='ifarule'
'ifarule' with letters rearranged gives 'FAILURE'.
(Other definitions for failure that I've seen before include "Collapse; disaster" , "[OPPOSITE OF] success" , "Fiasco" , "Lack of success, or he who lacks it" , "No-hoper" .)