The first cause for crime (7)
I believe the answer is:
treason
'for crime' is the definition.
(treason is a kind of crime)
'the first cause' is the wordplay.
'the first' becomes 't' (1st letter of 'the').
'cause' becomes 'reason' (I've seen this before).
't'+'reason'='TREASON'
(Other definitions for treason that I've seen before include "Betrayal of one's nation" , "Betrayal (of one's country)" , "Crime of treachery against the state" , "Offence against state" , "Crime agains one's country" .)