Unit of money (3)
I believe the answer is:
one
'unit' is the definition.
(synonyms)
'of money' is the wordplay.
'of' says the answer is hidden in the clue.
'ONE' can be found hidden inside '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" .)