All's not all we hear (3)
I believe the answer is:
sum
'all's' is the definition.
(I've seen this before)
'not all we hear' is the wordplay.
'not all' becomes 'some' ('some' is the opposite to 'all').
'we hear' shows a homophone (sound like).
'some' sounds like 'SUM'.
(Other definitions for sum that I've seen before include "Amount of money; total" , "Amount, aggregate" , "Whole" , "Amount, of money say" , "All one has" .)