One giving friendly proof of literacy (3,3)
I believe the answer is:
pen pal
'one giving friendly proof of literacy' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for pen pal that I've seen before include "Friend you know only through correspondence" , "juvenile writer" , "Mail friend" , "A friend who communicates through correspondence" , "Friend who communicates in writing" .)