Breath-sweetener is something nutty, we hear (6)
I believe the answer is:
cachou
'breath-sweetener' is the definition.
(old-fashioned type of breath-sweetener)
'something nutty we hear' is the wordplay.
'something nutty' becomes 'cashew' (cashew is a type of nut).
'we hear' indicates a 'sounds like' (homophone) clue.
'cashew' sounds like 'CACHOU'.
'is' acts as a link.
(Other definitions for cachou that I've seen before include "pill to sweeten the breath" , "Scented lozenge to sweeten the breath" , "Breath-sweetening lozenge" .)