Jubilant cry (3)
'jubilant cry' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for yes that I've seen before include "Indeed" , "OK" , "Affirmative reply" , "Positive response" , "Confirmation" .)
(Other definitions for eureka that I've seen before include "Archimedes's call of triumph" , "Discovery announcement" , "Archimedes' shout of triumph - re a uke (anag)" , "county seat in 23 [CALIFORNIA]" , "Bingo!" .)