Sum to put back (3,2)
I believe the answer is:
tot up
'sum' is the definition.
('tot up' can be a synonym of 'sum')
'to put back' is the wordplay.
'back' is a reversal indicator.
'put' back-to-front is 'tup'.
'to'+'tup'='TOT UP'
(Other definitions for tot up that I've seen before include "Reckon" , "Tally" , "Add (some numbers)" , "Summate" , "Determine the sum of" .)