Betrayal Tristan's first motivation (7)
I believe the answer is:
treason
'betrayal' is the definition.
(I know that betrayal can be written as treason)
'tristan's first motivation' is the wordplay.
'tristan's first' becomes 't' (1st letter of 'tristan').
'motivation' becomes 'reason' (reason is a kind of motivation).
't'+'reason'='TREASON'
(Other definitions for treason that I've seen before include "Betrayal of one's nation" , "Disloyalty (to one's country)" , "rebellion" , "Senator (anag)" , "Crime of national betrayal" .)