Add to set brought back (3,2)
I believe the answer is:
tot up
'add' is the definition.
('tot up' can be a synonym of 'add')
'to set brought back' is the wordplay.
'set' becomes 'put' ('put' can be a synonym of 'set').
'brought back' says the letters should be written backwards.
'put' back-to-front is 'tup'.
'to'+'tup'='TOT UP'
(Other definitions for tot up that I've seen before include "Do summer job" , "Reckon" , "Add together (driving offences)" , "Add (some numbers)" , "Sum - total" .)