Express in song (3)
I believe the answer is:
air
This clue is a double definition.
'express' is the first definition.
(I've seen this before)
'song' is the second definition.
(thesaurus)
'in' acts as a link.
(Other definitions for air that I've seen before include "Broadcast; tune" , "- brick; - bed; - brush" , "Tune - mood - atmosphere" , "in which birds fly around" , "See 21" .)