Gladden dumbstruck participants with supplementary piece (8)
I believe the answer is:
addendum
'supplementary piece' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'gladden dumbstruck participants with' is the wordplay.
'with' indicates a hidden word.
'ADDENDUM' can be found hidden inside 'gladden dumbstruck participants'.
Can you help me to learn more?
(Other definitions for addendum that I've seen before include "Missing passage to be appended" , "Dud ad-men dealt with in end of book" , "Postscript" , "Text added at the end" , "Addition at end of book" .)