Small child raised to do simple arithmetic (3,2)
I believe the answer is:
tot up
'to do simple arithmetic' is the definition.
'small child raised' is the wordplay.
'small child' becomes 'tot' (I've seen this before).
'raised' becomes 'up' ('up' can be similar in meaning to 'raised').
'tot'+'up'='TOT UP'
(Other definitions for tot up that I've seen before include "Sum - total" , "Reckon" , "Do summer job" , "Add together (driving offences)" , "Tally" .)