Sums worked out, not a sum (7)
I believe the answer is:
amounts
'sums' is the definition.
('amount' can be a synonym of 'sum')
'worked out not a sum' is the wordplay.
'worked out' indicates an anagram.
'not'+'a'+'sum'='notasum'
'notasum' with letters rearranged gives 'AMOUNTS'.
(Other definitions for amounts that I've seen before include "Totals, sums" , "Supplies" , "Total numbers or quantities" , "Measures" .)