Classmates or upper-class mates? (4,5)
I believe the answer is:
peer group
'classmates' is the definition.
(classmates are one's peers)
'upper-class mates?' is the wordplay.
'upper class' becomes 'peer' (peer is a kind of upper class).
'mates?' becomes 'group' (mates is a kind of group).
'peer'+'group'='PEER GROUP'
'or' acts as a link.
(Other definitions for peer group that I've seen before include "Classmates, say" , "People who are associated with and of equal status" , "contemporaries - with 15d" , "those of similar age" , "People of a certain age" .)