Cutter does sums out loud (4)
I believe the answer is:
adze
'cutter' is the definition.
(adze is a kind of cutter)
'sums out loud' is the wordplay.
'sums' becomes 'adds' ('add' can be a synonym of 'sum').
'out loud' shows a homophone (sound like).
'adds' sounds like 'ADZE'.
'does' acts as a link.
(Other definitions for adze that I've seen before include "One used to cut" , "Edging tool" , "Wood-dressing tool" , "Something with sharp edge" , "Carpenter's tool sounds mathematical" .)