Amount married American sent over (3)
I believe the answer is:
sum
'amount' is the definition.
(I know that amount can be written as sum)
'married american sent over' is the wordplay.
'married' becomes 'm' (abbreviation, e.g. in genealogy).
'american' becomes 'US' (abbreviation for United States).
'sent over' says the letters should be written in reverse.
'm'+'us'='mus'
'mus' back-to-front is 'SUM'.
(Other definitions for sum that I've seen before include "Aggregate, total" , "Mathematical problem" , "Total, add" , "All one has" , "See 25" .)