Money: unknown quantity? Small (4)
I believe the answer is:
tiny
'small' is the definition.
(similar in meaning)
'money unknown quantity?' is the wordplay.
'money' becomes 'tin' (old-fashioned slang term for money).
'unknown quantity?' becomes 'y' (algebra).
'tin'+'y'='TINY'
(Other definitions for tiny that I've seen before include "Minuscule" , "Minute, diminutive" , "- - Tim (Dickens)" , "Little" , "Miniscule" .)