Missing, perhaps stolen, mostly (4)
I believe the answer is:
lost
'missing' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for lost that I've seen before include "Mislaid or strayed" , "TV show" , "Unable to find the way forward" , "Not recognising one's whereabouts" , "Mogul empire official; rich person" .)