No elf can become 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 can' is the wordplay.
'can' indicates an anagram (I've seen 'canned' mean this (canned can mean drunk)).
'no'+'elf'='noelf'
'noelf' with letters rearranged gives 'FELON'.
'become' acts as a link.
(Other definitions for felon that I've seen before include "Mighty" , "A dealer in textile" , "Serious criminal" , "Wrong-doer" , "Convicted criminal" .)