Crime trends are halved when working (7)
I believe the answer is:
treason
'crime' is the definition.
(treason is a kind of crime)
'trends are halved when working' is the wordplay.
'are halved' means to take half the letters.
'when' becomes 'as'.
'working' becomes 'on'.
'trends' cut in half is 'tre'.
'tre'+'as'+'on'='TREASON'
(Other definitions for treason that I've seen before include "Act of betrayal of a government" , "Mutiny" , "subversion" , "rebellion" , "Crime of treachery against the state" .)