Passage in melody containing new line (6)
I believe the answer is:
tunnel
'passage' is the definition.
(tunnel is a kind of passage)
'melody containing new line' is the wordplay.
'melody' becomes 'tune' (synonyms).
'containing' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'line' becomes 'l'.
'tune' enclosing 'n' is 'tunne'.
'tunne'+'l'='TUNNEL'
'in' acts as a link.
(Other definitions for tunnel that I've seen before include "See 1" , "Underground bore" , "See 6" , "See 7" , "Underground passage for road or railway" .)