Ultimately just cause for betrayal (7)
I believe the answer is:
treason
'for betrayal' is the definition.
(synonyms)
'ultimately just cause' is the wordplay.
'ultimately' indicates one should take the final letters.
'cause' becomes 'reason' (I've seen this before).
The last letter of 'just' is 't'.
't'+'reason'='TREASON'
(Other definitions for treason that I've seen before include "Crime of national betrayal" , "Crime of treachery against the state" , "One rats (anag)" , "subversion" , "attempting to overthrow the king" .)