One of those with links to Fingers - little better than the banks, in retrospect (4)
I believe the answer is:
nail
'one' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for nail that I've seen before include "Growth on digit" , "Horny covering of the digits" , "Finger end; fasten" , "Expose as a lie" , "Claw" .)