Old boy, one who works in restaurant as supervisor (8)
I believe the answer is:
observer
'old boy one who works in restaurant as supervisor' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'old boy one who works' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'one who works' becomes 'server' (I am not sure about this - if you are sure you should believe this answer much more).
'ob'+'server'='OBSERVER'
Can you help me to learn more?
(Other definitions for observer that I've seen before include "He sees" , "One watching attentively" , "Viewer" , "Looker-on" , "One watching, evaluating" .)