What letter-writers often claim to be (7)
I believe the answer is:
sincere
'often claim to be' is the definition.
I can't judge whether this defines the answer.
'what letter-writers' is the wordplay.
I cannot quite see how this works, but
'letter' could be 'sin' (sin is a kind of letter of the alphabet) and 'sin' is present in the answer.
The remaining letters 'cere' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sincere that I've seen before include "True" , "Unfeigned, genuine" , "Showing genuine feelings" , "Lacking in deceit" , "straightforward" .)