To back us with a million takes money! (3)
I believe the answer is:
sum
'money' is the definition.
(I've seen this before)
'to back us with a million' is the wordplay.
'to back' is a reversal indicator.
'with' is a charade indicator (letters next to each other).
'a million' becomes 'm' (abbreviation).
'us' reversed gives 'su'.
'su'+'m'='SUM'
'takes' acts as a link.
(Other definitions for sum that I've seen before include "maths problem" , "Add together" , "Arithmetical problem" , "Amount of money; add" , "Total, add" .)