PC in peril breaking law (9)
I believe the answer is:
principle
'law' is the definition.
(I know that principle is a type of law)
'pc in peril breaking' is the wordplay.
'breaking' indicates anagramming the letters.
'pc'+'in'+'peril'='pcinperil'
'pcinperil' is an anagram of 'PRINCIPLE'.
(Other definitions for principle that I've seen before include "General guiding rule" , "Law of nature, rule of conduct" , "In which, theoretically" , "Rule of personal conduct" , "Tenet" .)