Clue: Extra piece the author put in letter, section of second letter or third (3)
Answer: PSI
Explanation: definition: a third [letter] in the context of the surface 
PS=postscript="Extra piece… put in letter", with I="the author"
 
"section of second letter" – the letters PSI form a section of the Greek letter E-PSI-LON in the grid, with letters from 17ac and 20ac