Single thing (4)
I think it could be either:
'single thing' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for unit that I've seen before include "Subdivision of a military group" , "Individual thing; one" , "Entity, system; component group" , "Single whole as part of a larger whole" , "Quantity chosen as standard, or individual thing" .)
(Other definitions for item that I've seen before include "Piece in paper" , "Loving couple" , "Unit; detail" , "Object, detail" , "One thing among many" .)