Beginners in algebra dread doing stupid sums (4)
I believe the answer is:
'sums' is the definition.
('add' can be a synonym of 'sum')
'beginners in algebra dread doing stupid' is the wordplay.
'beginners in' says to take the initial letters.
The first letters of 'algebra dread doing stupid' is 'ADDS'.
(Other definitions for adds that I've seen before include "Does summer job" , "Supplements" , "Calculates the sum" , "Aggregates or makes further remark" , "Puts in; tacks on" .)