Copper pen in 3-D (5)
I believe the answer is:
cubic
'3-d' is the definition.
(I've seen this in another clue)
'copper pen' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'pen' becomes 'bic' (pen manufacturer).
'cu'+'bic'='CUBIC'
'in' is the link.
(Other definitions for cubic that I've seen before include "puzzling sort of equation" , "With six equal faces" , "Of the third power (mathematically)" , "Like sugar lumps" , "The first C in cc" .)