On the surface, it's hard to sell alternate title internally (6)
I believe the answer is:
enamel
'on the surface it's hard' is the definition.
I can't tell whether this defines the answer.
'sell alternate title internally' is the wordplay.
'sell alternate' becomes 'el' (I can't justify this - if you can you should give a lot more credence to this answer).
'title' becomes 'name' (I've seen this in other clues).
'internally' is an insertion indicator.
'el' placed around 'name' is 'ENAMEL'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for enamel that I've seen before include "Hard white substance covering the crown of a tooth" , "Hard substance covering teeth" , "Tooth surface" , "Coating for a tooth" , "Shiny layer on surface" .)