Solid copper pen manufacturer (5)
I believe the answer is:
cubic
'solid' is the definition.
('cubic' can be similar in meaning to 'solid')
'copper pen manufacturer' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'pen manufacturer' becomes 'bic' (pen brand).
'cu'+'bic'='CUBIC'
(Other definitions for cubic that I've seen before include "puzzling sort of equation" , "Die-shaped" , "Relating to volume" , "Shaped like a solid square" , "Having three dimensions" .)