Motive after time is subversion (7)
I believe the answer is:
treason
'is subversion' is the definition.
(I've seen this before)
'motive after time' is the wordplay.
'motive' becomes 'reason' (reason is a kind of motive).
'after' says to put letters next to each other.
'time' becomes 't' (abbreviation).
'reason' after 't' is 'TREASON'.
(Other definitions for treason that I've seen before include "Act of betrayal of a government" , "rebellion" , "Disloyalty (to one's country)" , "attempting to overthrow the king" , "Gunpowder Plot, say" .)