Sums up argument decisively, distressing school heads (4)
I believe the answer is:
adds
'sums up' is the definition.
('add' can be a synonym of 'sum up')
'argument decisively distressing school heads' is the wordplay.
'heads' suggests taking the first letters.
The initial letters of 'argument decisively distressing school' is 'ADDS'.
(Other definitions for adds that I've seen before include "Aggregates or makes further remark" , "Tots up" , "Sums, totals" , "Finds the sum (4)," , "Supplements" .)