Nurse - don't open camel coat (6)
I believe the answer is:
Here is my best explanation:
'coat' is the definition.
(enamel is a kind of coat)
'nurse don't open camel' is the wordplay.
'nurse' becomes 'en' (abbreviation for enrolled nurse).
'don't open' means to remove the first letter (the letter which opens the word is removed).
'camel' with its first letter taken away is 'amel'.
(Other definitions for enamel that I've seen before include "Crown Protector?", "hard stuff", "art form", "Decorative or protective covering on metal or glass.", "nail polish", "protection for canines?", "Hard white substance covering the crown of a tooth", "Shiny coat as on teeth", "Covering on teeth" and "Glossy coating".)