Perfect ending and motive for a crime (7)
I believe the answer is:
treason
'a crime' is the definition.
(treason is a kind of crime)
'perfect ending and motive' is the wordplay.
'ending' suggests the final letters.
'and' is a charade indicator (letters next to each other).
'motive' becomes 'reason' (reason is a kind of motive).
The last letter of 'perfect' is 't'.
't'+'reason'='TREASON'
'for' is the link.
(Other definitions for treason that I've seen before include "Lese-majesty" , "Betraying one's country" , "rebellion" , "Disloyalty (to one's country)" , "Act of betrayal of a government" .)