So you've left school, dear fellow! (3,3)
I believe the answer is:
old boy
'fellow' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for old boy that I've seen before include "my dear chap!" , "Former member of a school" , "Male former pupil" , "Mate" , "Alumnus (with network)" .)