Terribly mean to the Spanish - that's hard (6)
I believe the answer is:
enamel
'that's hard' is the definition.
I can't tell whether this definition defines the answer.
'terribly mean to the spanish' is the wordplay.
'terribly' indicates an anagram.
'to the spanish' becomes 'el' ('the' in Spanish).
'mean' is an anagram of '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" .)