Nothing in money supply? A very small amount (4)
I believe the answer is:
atom
'a very small amount' is the definition.
(I've seen this in another clue)
'nothing in money supply?' is the wordplay.
'nothing in' becomes 'ato' (I can't explain this - if you can you should give a lot more credence to this answer).
'money supply?' becomes 'M' (economics abbreviation).
'ato'+'m'='ATOM'
Can you help me to learn more?
(Other definitions for atom that I've seen before include "Moat (anag.)" , "Nucleus plus electrons" , "A really tiny piece of something" , "Small particle of matter" , "Bit of make-up" .)