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 defines the answer.
'terribly mean to the spanish' is the wordplay.
'terribly' indicates an anagram.
'to the spanish' becomes 'el' ('the' in Spanish).
'mean' with letters rearranged 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" .)