Nurse - don't open camel coat (6)
I believe the answer is:
enamel
'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'.
'en'+'amel'='ENAMEL'
(Other definitions for enamel that I've seen before include "Hard material" , "Hard substance covering teeth" , "hard cover" , "Outer tooth material" , "protective coat" .)