Signed up or listened out (8)
I believe the answer is:
enlisted
'signed up' is the definition.
(enlisting is a kind of signing up)
'listened out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'listened' anagrammed gives 'ENLISTED'.
'or' acts as a link.
(Other definitions for enlisted that I've seen before include "Listened (anag.)" , "Enrolled" , "On a roll" , "Joined, having listened" , "Joined forces" .)