Agent's ultimate motivation is betrayal (7)
I believe the answer is:
treason
'is betrayal' is the definition.
(I've seen this before)
'agent's ultimate motivation' is the wordplay.
'agent's ultimate' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'motivation' becomes 'reason' (reason is a kind of motivation).
't'+'reason'='TREASON'
(Other definitions for treason that I've seen before include "Senator (anag)" , "Criminal attempt to overthrow a government" , "Mutiny" , "Stare on (anag.)" , "Betrayal of the state" .)