School peer is an excellent friend (9)
I believe the answer is:
classmate
'school peer' is the definition.
'classmate' can be an answer for 'peer' (I've seen this in another clue). I'm not certain of the 'school' bit.
'an excellent friend' is the wordplay.
'an excellent' becomes 'class' (I've seen this before).
'friend' becomes 'mate' (mate is a kind of friend).
'class'+'mate'='CLASSMATE'
'is' is the link.
Can you help me to learn more?
(Other definitions for classmate that I've seen before include "someone from school?" , "one of the others being in form" , "School associate" , "Fellow student" , "Fellow pupil" .)