Northerner's specific motive for being disloyal? (7)
I believe the answer is:
treason
'being disloyal?' is the definition.
'northerner's specific motive' is the wordplay.
'northerner's specific' becomes 't' (t' is a northern English dialect form of 'the').
'motive' becomes 'reason' (reason is a kind of motive).
't'+'reason'='TREASON'
'for' is the link.
(Other definitions for treason that I've seen before include "attempting to overthrow the king" , "Serious crime" , "Disloyalty (to one's country)" , "Senator (anag)" , "Crime of national betrayal" .)