It's a crime - so ran when disturbed (5)
I believe the answer is:
arson
'it's a crime' is the definition.
(crime of illegally starting a fire)
'so ran when disturbed' is the wordplay.
'when disturbed' is an anagram indicator.
'so'+'ran'='soran'
'soran' anagrammed gives 'ARSON'.
(Other definitions for arson that I've seen before include "major crime" , "Felonious burning of property" , "Crime of fire raising" , "Fire requiring forensic study" , "It may get you in court" .)