Listen out when quiet (6)
I believe the answer is:
silent
'quiet' is the definition.
('silent' can be similar in meaning to 'quiet')
'listen out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'listen' anagrammed gives 'SILENT'.
'when' is the link.
(Other definitions for silent that I've seen before include "Strange and astonishing thing" , "Lacking sound" , "Without sound" , "Mute" , "Anagram of 7 Down" .)