Disloyalty Tories' principal motivation (7)
I believe the answer is:
treason
'disloyalty' is the definition.
(I know that treason is a type of disloyalty)
'tories' principal motivation' is the wordplay.
'tories' principal' 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'
Can you help me to learn more?
(Other definitions for treason that I've seen before include "Senator (anag)" , "attempting to overthrow the king" , "Disloyalty (to one's country)" , "LEse-majestE" , "Betrayal of the state" .)