They make cubes - but nothing's squared? (3)
I believe the answer is:
oxo
'they make cubes' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'nothing's squared?' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'squared?' becomes 'xo' (I am not sure about this - if you are sure you should believe this answer much more).
'o'+'xo'='OXO'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for oxo that I've seen before include "Stock cube brand" , "Proprietary brand of beef cube" .)