Stated function for token (4)
I believe the answer is:
sign
'token' is the definition.
(I've seen this before)
'stated function' is the wordplay.
'stated' indicates a 'sounds like' (homophone) clue.
'function' becomes 'sine' (mathematical function).
'sine' is a homophone of 'SIGN'.
'for' acts as a link.
(Other definitions for sign that I've seen before include "Put one's name to a document" , "Authorise with a name" , "Engage by written agreement" , "Manifestation" , "Public notice; portent" .)