No elf could be a criminal! (5)
I believe the answer is:
felon
'a criminal' is the definition.
(I know that criminal can be written as felon)
'no elf could be' is the wordplay.
'could be' indicates anagramming the letters.
'no'+'elf'='noelf'
'noelf' anagrammed gives 'FELON'.
(Other definitions for felon that I've seen before include "Person who has committed a serious crime" , "Serious criminal" , "criminal from Texas" , "sore" , "Law-breaker" .)