Someone who looks old alongside British waiter (8)
I believe the answer is:
observer
'someone who looks' is the definition.
(someone observing or looking at things)
'old alongside british waiter' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'alongside' says to put letters next to each other.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'waiter' becomes 'server' (synonyms).
'o'+'b'+'server'='OBSERVER'
(Other definitions for observer that I've seen before include "Someone watching" , "Watcher, spectator" , "Viewer" , "Looker-on" , "'Onlooker, spectator (8)'" .)