Visitor is supposed to be announced (5)
I believe the answer is:
guest
'visitor' is the definition.
(someone visiting a place)
'supposed to be announced' is the wordplay.
'supposed' becomes 'guessed' ('guess' can be a synonym of 'suppose').
'to be announced' indicates a 'sounds like' (homophone) clue.
'guessed' is a homophone of 'GUEST'.
'is' is the link.
(Other definitions for guest that I've seen before include "Visitant" , "Caller" , "Conjectured audibly" , "company" , "Recipient of hospitality" .)