Words fit to air on TV programme (5,4)
I believe the answer is:
theme song
'tv programme' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'words fit to air on' is the wordplay.
'words fit' becomes 'theme' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to air' becomes 'song' (air can mean a piece of music).
'on' says to put letters next to each other.
'theme'+'song'='THEME SONG'
Can you help me to learn more?
(Other definitions for theme song that I've seen before include "Signature tune" , "Give me Sunshine" , "Characteristic melody often repeated in musical" .)