On paper, perhaps, the hard stuff (5)
I believe the answer is:
emery
This could be a double definition.
'on paper' is the first definition.
'emery' can be an answer for 'paper' (I have seen 'Abrasive paper ' mean 'emery' so perhaps 'paper' could also mean 'emery'). I am not sure about the 'on' bit.
'the hard stuff' is the second definition.
(I've seen this before)
'perhaps' acts as a link.
Can you help me to learn more?
(Other definitions for emery that I've seen before include "Abrasive form of corundum" , "Abrasive powder" , "Coating on paper used as an abrasive" , "hard stuff" , "Hard grey-black mineral used as an abrasive" .)