The principal inducement for betrayal (7)
I believe the answer is:
treason
'betrayal' is the definition.
(synonyms)
'the principal inducement' is the wordplay.
'principal' says to take the initial letters.
'inducement' becomes 'reason'.
The first letter of 'the' is 't'.
't'+'reason'='TREASON'
'for' acts as a link.
(Other definitions for treason that I've seen before include "One rats (anag)" , "Crime of treachery against the state" , "Betrayal of the state" , "Stare on (anag.)" , "Betraying one's country" .)