Public placard’s function, according to report (4)
I believe the answer is:
sign
'public placard's' is the definition.
(placard or poster in a public place)
'function according to report' is the wordplay.
'function' becomes 'sine' (mathematical function).
'according to report' indicates a 'sounds like' (homophone) clue.
'sine' sounds like 'SIGN'.
(Other definitions for sign that I've seen before include "Signal, gesture" , "Token - symbol - omen - write one's name" , "part of the zodiac" , "Notice; omen" , "house" .)