Transmitted about one line without noise (6)
I believe the answer is:
silent
'without noise' is the definition.
(silence is the absence of noise)
'transmitted about one line' is the wordplay.
'transmitted' becomes 'sent' (sending is a kind of transmitting).
'about' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'line' becomes 'l' (used when specifying particular lines from a poem).
'i'+'l'='il'
'sent' placed around 'il' is 'SILENT'.
(Other definitions for silent that I've seen before include "Muted" , "Anagram of 7 Down" , "still" , "Listen, being speechless after 10 across" , "Strange and astonishing thing" .)