Crime with a tip-top motive? (7)
I believe the answer is:
treason
'crime' is the definition.
(I know that treason is a type of crime)
'with a tip-top motive?' is the wordplay.
'with' says to put letters next to each other.
'a tip-top' becomes 't' (top of the word 'tip-').
'motive?' becomes 'reason' (reason is a kind of motive).
't'+'reason' is 'TREASON'.
(Other definitions for treason that I've seen before include "Criminal attempt to overthrow a government" , "Betrayal of one's nation" , "Betrayal (of one's country)" , "Perfidy" , "Mutiny" .)