I must leave firm, betrayed (4)
I believe the answer is:
sold
'betrayed' is the definition.
('sell' can be a synonym of 'betray')
'i must leave firm' is the wordplay.
'must leave' suggests deleting specific letters.
'firm' becomes 'solid' ('solid' can be a synonym of 'firm').
'solid' with 'i' taken out is 'SOLD'.
(Other definitions for sold that I've seen before include "Vended" , "Flogged" , "Disposed of to a purchaser" , "Disposed of for money" , "Betrayed for money" .)