Hardest human tissue (6)
I believe the answer is:
enamel
'hardest human tissue' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for enamel that I've seen before include "Decorative or protective covering on metal or glass." , "found on crowns" , "Hard shiny coating on surface" , "Dental material" , "hard cover" .)