Defendant's conclusion, justification for crime (7)
I believe the answer is:
treason
'for crime' is the definition.
(treason is a kind of crime)
'defendant's conclusion justification' is the wordplay.
'conclusion' says to take the final letters.
'justification' becomes 'reason' (I've seen this before).
The final letter of 'defendant' is 't'.
't'+'reason'='TREASON'
(Other definitions for treason that I've seen before include "LEse-majestE" , "Gunpowder Plot, say" , "subversion" , "Perfidy" , "Serious crime" .)