Betrayal Tristan’s principal motivation (7)
I believe the answer is:
treason
'betrayal' is the definition.
('treason' can be a synonym of 'betrayal')
'tristan's principal motivation' is the wordplay.
'principal' says to take the initial letters.
'motivation' becomes 'reason' (reason is a kind of motivation).
The initial letter of 'tristans' is 't'.
't'+'reason'='TREASON'
(Other definitions for treason that I've seen before include "Lese-majesty" , "Criminal attempt to overthrow a government" , "Crime of treachery against the state" , "subversion" , "Crime of disloyalty" .)