Motive behind West End crime (7)
I believe the answer is:
treason
'crime' is the definition.
(I know that treason is a type of crime)
'motive behind west end' is the wordplay.
'motive' becomes 'reason' (reason is a kind of motive).
'behind' means one lot of letters go next to another.
'end' suggests the final letters.
The final letter of 'west' is 't'.
'reason' put after 't' is 'TREASON'.
(Other definitions for treason that I've seen before include "Crime of treachery against the state" , "Criminal attempt to overthrow a government" , "Betrayal (of one's country)" , "LEse-majestE" , "Act of betrayal of a government" .)