Reportedly, not the whole amount (3)
I believe the answer is:
sum
'whole amount' is the definition.
(I've seen this in another clue)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sum that I've seen before include "Amount of money; total" , "Amount, of money say" , "maths problem" , "pupils find it a problem" , "All one has" .)