Cast are dummies, no offence (12)
I believe the answer is:
misdemeanour
'offence' is the definition.
(I know that misdemeanour is a type of offence)
'cast are dummies no' is the wordplay.
'cast' indicates an anagram (letters are dispersed or cast around).
'are'+'dummies'+'no'='aredummiesno'
'aredummiesno' anagrammed gives 'MISDEMEANOUR'.
(Other definitions for misdemeanour that I've seen before include "Minor wrongdoing" , "Minor violation" , "Minor offence" .)