Witness waiter, perhaps, supporting old boy (8)
I believe the answer is:
observer
'witness' is the definition.
(I've seen this before)
'waiter perhaps supporting old boy' is the wordplay.
'waiter perhaps' becomes 'server' (someone who serves diners).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'server' put after 'ob' is 'OBSERVER'.
(Other definitions for observer that I've seen before include "'Onlooker, spectator (8)'" , "monitor" , "Sunday newspaper" , "Spectator - or another weekly publication!" , "He sees" .)