One sending letters to China? (3,3)
I believe the answer is:
pen pal
'one sending letters' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'to china?' is the wordplay.
'to' becomes 'pen' (I can't justify this - if you can you should give a lot more credence to this answer).
'china?' becomes 'pal' (china plate is Cockney rhyming slang for mate).
'pen'+'pal'='PEN PAL'
Can you help me to learn more?
(Other definitions for pen pal that I've seen before include "Friend who communicates in writing" , "American friend via correspondence" , "A friendly correspondent" , "foreign correspondent" , "Friend you know only through correspondence" .)