Not a sum, somehow, but sums (7)
I believe the answer is:
amounts
'sums' is the definition.
('amount' can be a synonym of 'sum')
'not a sum somehow' is the wordplay.
'somehow' is an anagram indicator.
'not'+'a'+'sum'='notasum'
'notasum' is an anagram of 'AMOUNTS'.
'but' acts as a link.
(Other definitions for amounts that I've seen before include "Supplies" , "Total numbers or quantities" , "Measures" , "Totals, sums" .)