Time and motive for a serious crime (7)
I believe the answer is:
treason
'a serious crime' is the definition.
(I've seen this before)
'time and motive' is the wordplay.
'time' becomes 't'.
'and' means one lot of letters go next to another.
'motive' becomes 'reason' (reason is a kind of motive).
't'+'reason'='TREASON'
'for' is the link.
(Other definitions for treason that I've seen before include "Deliberate act of betrayal" , "LEse-majestE" , "Crime of national betrayal" , "Perfidy" , "attempting to overthrow the king" .)