Terribly mean to the Spanish - that's hard (6)
I believe the answer is:
enamel
'that's hard' is the definition.
I can't judge whether this definition defines the answer.
'terribly mean to the spanish' is the wordplay.
'terribly' indicates anagramming the letters.
'to the spanish' becomes 'el' ('the' in Spanish).
'mean' anagrammed gives 'enam'.
'enam'+'el'='ENAMEL'
Can you help me to learn more?
(Other definitions for enamel that I've seen before include "Decorative inlay" , "Hard material" , "Hard type of paint" , "Outer tooth material" , "Smooth hard paint" .)