Betrayed spouse, principally with ex (4)
I believe the answer is:
sold
'betrayed' is the definition.
('sell' can be a synonym of 'betray')
'spouse principally with ex' is the wordplay.
'principally' suggests taking the first letters.
'with ex' becomes 'old' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The first letter of 'spouse' is 's'.
's'+'old'='SOLD'
Can you help me to learn more?
(Other definitions for sold that I've seen before include "Flogged" , "Marketed" , "Disposed of for money" , "Betrayed for money" , "Vended" .)