Material appeared hard in earth (9)
I believe the answer is:
camelhair
'material' is the definition.
(camelhair is a kind of material)
'appeared hard in earth' is the wordplay.
'appeared' becomes 'came' (I've seen this before).
'hard' becomes 'h'.
'in' means one lot of letters goes inside another.
'earth' becomes 'lair' (I've seen this before**).
'h' put into 'lair' is 'lhair'.
'came'+'lhair'='CAMELHAIR'
(Other definitions for camelhair that I've seen before include "Expensive cloth from coat of desert carrier" , "Material for artists' brushes" .)