Lest gain becomes a crime... (8)
I believe the answer is:
stealing
'a crime' is the definition.
(I know that stealing is a type of property crime)
'lest gain becomes' is the wordplay.
'becomes' is an anagram indicator.
'lest'+'gain'='lestgain'
'lestgain' with letters rearranged gives 'STEALING'.
(Other definitions for stealing that I've seen before include "Illegally removing" , "Thieving" , "Taking illegally" , "Theft" , "Robbing" .)