The whole amount — or a little, we’re told (3)
I believe the answer is:
sum
'the whole amount' is the definition.
(I've seen this in another clue)
'a little we're told' is the wordplay.
'a little' becomes 'some' (both can mean a small amount).
'we're told' shows a homophone (sound like).
'some' sounds like 'SUM'.
'or' is the link.
(Other definitions for sum that I've seen before include "Total; problem" , "Quantity of money" , "Arithmetical problem" , "Arithmetic calculation" , "All one has" .)