The amount can upset us a lot (3)
I believe the answer is:
sum
'the amount' is the definition.
('sum' can be a synonym of 'amount')
'upset us a lot' is the wordplay.
'upset' says the letters should be written in reverse.
'a lot' becomes 'm' (**).
'us' reversed gives 'su'.
'su'+'m'='SUM'
'can' is the link.
(Other definitions for sum that I've seen before include "Whole" , "Add together" , "Total; problem" , "See 25" , "Amount of money; total" .)