Exclamation of discovery (6)
I believe the answer is:
eureka
'exclamation of discovery' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for eureka that I've seen before include "Cry of joy on finding something" , "announced by Archimedes" , "Triumphant cry of Greek" , "county seat in 23 [CALIFORNIA]" , ""I've found it!" (Archimedes)" .)