The irony is, spectator is missing cast (5)
I believe the answer is:
trope
'the irony' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe you can see an association between them that I can't see?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for trope that I've seen before include "Figurative use of a word or expression; motif" , "Word or expression used in a figurative way" , "figuratively speaking" , "Recurrent theme, motif" , "literary effect" .)