Down-and-out, one omitted from file (6)
I believe the answer is:
dosser
'down-and-out' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'one omitted from file' is the wordplay.
'one' becomes 'i' (Roman numeral).
'omitted from' indicates named letters should be taken away.
'file' becomes 'dossier' (both can mean a collection of information).
'dossier' with 'i' removed is 'DOSSER'.
Can you help me to learn more?
(Other definitions for dosser that I've seen before include "Idler, layabout" , "Homeless person" , "Someone in cheap lodgings" , "Idle person" , "Lazy person who does nothing" .)