The first excuse for disloyalty (7)
I believe the answer is:
treason
'for disloyalty' is the definition.
(treason is a kind of disloyalty)
'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'
(Other definitions for treason that I've seen before include "Stare on (anag.)" , "Act of betrayal of a government" , "Crime of disloyalty" , "subversion" , "Senator (anag)" .)