Copper pen in form of box? (5)
I believe the answer is:
cubic
'in form of box?' is the definition.
'copper pen' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'pen' becomes 'bic' (pen manufacturer).
'cu'+'bic'='CUBIC'
(Other definitions for cubic that I've seen before include "Relating to volume" , "Shaped like a solid square" , "Having six square sides" , "Equation involving third powers" , "Die-shaped" .)