Serve, wait upon (6)
I believe the answer is:
attend
I believe this is a double definition.
'serve' is the first definition.
(I know that serve can be written as attend)
'wait upon' is the second definition.
(I've seen this in another clue)
(Other definitions for attend that I've seen before include "Wait upon or be present" , "Be present at, or accompany" , "Pay heed" , "Go regularly to, church perhaps" , "Be present at, say meeting" .)