Betrayal Taliban's principal motivation (7)
I believe the answer is:
treason
'betrayal' is the definition.
(I know that betrayal can be written as treason)
'taliban's principal motivation' is the wordplay.
'principal' says to take the initial letters.
'motivation' becomes 'reason' (reason is a kind of motivation).
The first letter of 'taliban' is 't'.
't'+'reason'='TREASON'
(Other definitions for treason that I've seen before include "One's art? (anag)" , "Perfidy" , "Betrayal (of one's country)" , "Crime of national betrayal" , "One rats (anag)" .)