'Waiter and cook' advert's about right (7)
I believe the answer is:
steward
'waiter' is the definition.
(I've seen this before)
'cook advert's about right' is the wordplay.
'cook' becomes 'stew' (stewing is a kind of cooking**).
'advert' becomes 'ad' ('ad' can be a synonym of 'advert').
'about' means one lot of letters goes inside another.
'right' becomes 'r' (common abbreviation).
'ad' enclosing 'r' is 'ard'.
'stew'+'ard'='STEWARD'
'and' acts as a link.
(Other definitions for steward that I've seen before include "Flight attendant" , "Overseer" , "Sporting event supervisor" , "Attendant on ship - race official" , "Horse-racing official" .)