Dollar bill (6)
'dollar bill' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for one that I've seen before include "Lowest cardinal number" , "Single, sole" , "Single and undivided" , "Single, unitary" , "Unity" .)
(Other definitions for single that I've seen before include "Just one - on one's own" , "Sort of person" , "Unattached" , "Ticket To Ride < : > hit" , "Not in a relationship" .)