Poster in which function's announced (4)
I believe the answer is:
sign
'poster' is the definition.
(thesaurus)
'function's announced' is the wordplay.
'function' becomes 'sine' (mathematical function).
'announced' indicates a 'sounds like' (homophone) clue.
'sine' is a homophone of 'SIGN'.
'in which' acts as a link.
(Other definitions for sign that I've seen before include "Show" , "house" , "Public notice; Leo, say" , "Communicate silently" , "Token - symbol - omen - write one's name" .)