Clue: Spy network endlessly stifling triumphant outburst (4,4)
Answer: MATA HARI
Explanation: Definition: Spy
An envelope ('stifling') of AHA ('triumphal outburst') in MATRI[x] ('network') minus its last letter ('endlessly'). Mata Hari, real name Margaretha Geertruide Zelle, was a Dutch exotic dancer, executed by a French firing squad during the First World War, as a spy for the Germans.