Turncoat's first motive for disloyalty (7)
I believe the answer is:
'for disloyalty' is the definition.
(treason is a kind of disloyalty)
'turncoat's first motive' is the wordplay.
'turncoat's first' becomes 't' (1st letter of 'turncoat').
'motive' becomes 'reason' (reason is a kind of motive).
(Other definitions for treason that I've seen before include "Senator (anag)" , "Betrayal of one's nation" , "Serious crime" , "Deliberate act of betrayal" , "subversion" .)