Part of the nameless protective coat (6)
I believe the answer is:
'protective coat' is the definition.
(I've seen this before)
'part of the nameless' is the wordplay.
'part of' indicates a hidden word.
'ENAMEL' is hidden within 'the nameless'.
(Other definitions for enamel that I've seen before include "Coating for a tooth" , "canine protection?" , "Vitrified coating for metal etc" , "sort of paint" , "Hard shiny surface as on teeth" .)