Fool with hard material (5)
I believe the answer is:
cloth
'material' is the definition.
(I know that material can be written as cloth)
'fool with hard' is the wordplay.
'fool' becomes 'clot' (I've seen this before).
'with' says to put letters next to each other.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'clot'+'h'='CLOTH'
(Other definitions for cloth that I've seen before include "Fabric, textile" , "ministers" , "The clergy" , "Woven material" , "Textiles" .)