One needs time and motivation for such a crime (7)
I believe the answer is:
treason
'such a crime' is the definition.
(treason is a kind of crime)
'one needs time and motivation' is the wordplay.
'one needs time' becomes 't' (I can't explain this - if you can you should believe this answer much more).
'and' says to put letters next to each other.
'motivation' becomes 'reason' (reason is a kind of motivation).
't'+'reason'='TREASON'
'for' is the link.
(Other definitions for treason that I've seen before include "Mutiny" , "Lese-majesty" , "Deliberate act of betrayal" , "Betraying one's country" , "Serious crime" .)