Betrothed lost navy revenue (6)
I believe the answer is:
fiance
'betrothed' is the definition.
(fiance is a kind of betrothed)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for fiance that I've seen before include "Prospective groom" , "Man engaged to marry" , "One engaged to marry" , "Engaged male" , "He's engaged to be married" .)