Certain quantity of money, we hear (4)
I believe the answer is:
some
'certain' is the definition.
('some' can be a synonym of 'certain')
'quantity of money we hear' is the wordplay.
'quantity of money' becomes 'sum' (I've seen this before).
'we hear' shows a homophone (sound like).
'sum' sounds like 'SOME'.
(Other definitions for some that I've seen before include "An undetermined number" , "Part" , "certain" , "A little; not a little" , "Undefined number" .)