No caps let luggage down! (5,4)
I believe the answer is:
lower case
'no' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'caps let luggage down' is the wordplay.
I cannot quite see how this works, but
'luggage' could be 'case' (eg a suitcase) and 'case' is present in the answer.
'down' could be 'lower' (synonyms) and 'lower' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lower case that I've seen before include "lacking capital" , "Unlike sentence, initially" , "small type" , "inappropriate in High Court?" , "in which I may never appear" .)