Breaking par, eight lead (8)
I believe the answer is:
graphite
'lead' is the definition.
(I've seen this before)
'breaking par eight' is the wordplay.
'breaking' indicates anagramming the letters.
'par'+'eight'='pareight'
'pareight' with letters rearranged gives 'GRAPHITE'.
(Other definitions for graphite that I've seen before include "Form of carbon used in pencil leads" , "Pig hater (anag)" , "Pencil-lead carbon" , "Writing material" , "Form of carbon used in pencils" .)