Crossword Genius

Sum to put back (3,2)

Ross

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" .)

I've seen this clue in The Times.
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