The original cause of disloyalty (7)
I believe the answer is:
treason
'disloyalty' is the definition.
(treason is a kind of disloyalty)
'the original cause' is the wordplay.
'original' suggests taking the first letters.
'cause' becomes 'reason' (I've seen this before).
The initial letter of 'the' is 't'.
't'+'reason'='TREASON'
'of' is the link.
(Other definitions for treason that I've seen before include "Disloyalty (to one's country)" , "Mutiny" , "Offence against state" , "LEse-majestE" , "Betrayal of the state" .)