Good French coating for unusually hard instrument (7)

'instrument' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'good french coating for unusually hard' is the wordplay.
I cannot really understand how this works, but
an anagram of 'hard' is 'dhra' which is located in the answer.
'good' could be 'bon' (I've seen this in other clues) and 'bon' is found within the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?