Don't arm a criminal willy-nilly (2,6)
I believe the answer is:
at random
'willy-nilly' is the definition.
(I've seen this before)
'don't arm a criminal' is the wordplay.
'criminal' is an anagram indicator.
'dont'+'arm'+'a'='dontarma'
'dontarma' with letters rearranged gives 'AT RANDOM'.
(Other definitions for at random that I've seen before include "unplanned" , "Indiscriminately" , "without purpose" , "Casually" , "Without method or conscious decision" .)