'Aha!', said solver, 'A stinker!' (6)
I believe the answer is:
eureka
'aha' is the definition.
(both can be said when discovering something)
I can't explain the remainder of the clue.
(Other definitions for eureka that I've seen before include "Exclamation of triumph" , "Archimedes' cry of triumph as he ran naked through the streets of Syracuse?" , "Archimedes's call of triumph" , "Shriek of discovery" , "Archimedes' shout of triumph - re a uke (anag)" .)