Opts out of appointment (4)
I believe the answer is:
post
'appointment' is the definition.
(I've seen this before)
'opts out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'opts' anagrammed gives 'POST'.
'of' acts as a link.
(Other definitions for post that I've seen before include "Prefix meaning after" , "Letters and parcels; job" , "Publicly display; send" , "Appointment (with the Royal Mail?)" , "(Holiday) communication" .)