Unit of money (3)
I believe the answer is:
one
'unit' is the definition.
(synonyms)
'of money' is the wordplay.
'of' indicates a hidden word.
'ONE' is hidden within 'money'.
(Other definitions for one that I've seen before include "joke" , "Single person or thing" , "Smallest whole number" , "This is singular" , "Half of two" .)