The first excuse for betrayal? (7)
I believe the answer is:
treason
'betrayal?' is the definition.
('treason' can be a synonym of 'betrayal')
'the first excuse' is the wordplay.
'the first' becomes 't' (1st letter of 'the').
'excuse' becomes 'reason' (I've seen this before).
't'+'reason'='TREASON'
'for' acts as a link.
(Other definitions for treason that I've seen before include "Crime of treachery against the state" , "Mutiny" , "Betraying one's country" , "Deliberate act of betrayal" , "Crime of disloyalty" .)