Sum to deposit, rising (3,2)
I believe the answer is:
tot up
'sum' is the definition.
('tot up' can be a synonym of 'sum')
'to deposit rising' is the wordplay.
'deposit' becomes 'put' (I've seen this in other clues).
'rising' says the letters should be written backwards (going up in a down clue).
'put' in reverse letter order is 'tup'.
'to'+'tup'='TOT UP'
(Other definitions for tot up that I've seen before include "Determine the sum of" , "Add together (driving offences)" , "Reckon" , "Tally" , "Count" .)