The glory that was Greek (5)
I believe the answer is:
kudos
This could be a double definition.
'the glory' is the first definition.
(I've seen this before)
'greek' is the second definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe there's an association between them I don't understand?
'that was' is the link.
Can you help me to learn more?
(Other definitions for kudos that I've seen before include "Laudation" , "Praise and honours" , "Honour - cachet" , "'Credit, honour or prestige (5)'" , "Great prestige" .)