Crossword Genius

What letter-writers often claim to be (7)

Ross

I believe the answer is:

sincere

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Globe and Mail.
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