Money? Back us with a thousand (3)
I believe the answer is:
sum
'money?' is the definition.
(I've seen this in other clues)
'back us with a thousand' is the wordplay.
'back' shows that the letters should be reversed in order.
'with' is a charade indicator (letters next to each other).
'a thousand' becomes 'm' (Roman numeral for a thousand).
'us' backwards is 'su'.
'su'+'m'='SUM'
(Other definitions for sum that I've seen before include "Amount, of money say" , "maths problem" , "Whole" , "Aggregate, total" , "Amount, aggregate" .)