Letter openers? (10)
I believe the answer is:
addressees
'openers?' is the definition.
Although both the answer and definition are plural nouns, I can't see how they can define each other.
'letter' is the wordplay.
I cannot really see how this works, but
'letter' could be 'see' (possible spelling of 'C' according to Chambers) and 'see' is found in the answer.
The remaining letters 'address' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for addressees that I've seen is " Group getting letters".)