Hard layer found in even a melon (6)
I believe the answer is:
enamel
'hard layer' is the definition.
'in even a melon' is the wordplay.
'in' indicates a hidden word.
'ENAMEL' can be found hidden inside 'even a melon'.
'found' is the link.
(Other definitions for enamel that I've seen before include "Glassy coating" , "canine protection?" , "Hard shiny surface as on teeth" , "sort of paint" , "protection for canines?" .)