Extra hard material (5)
I believe the answer is:
plush
'material' is the definition.
(I know that plush is a type of material)
'extra hard' is the wordplay.
'extra' becomes 'plus' (plus can mean more than expected).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'plus'+'h'='PLUSH'
(Other definitions for plush that I've seen before include "Fabric with a long velvety pile" , "Rich fabric" , "Fabric; richly luxurious" , "Cut pile, softer than velvet" .)