Demonstrate test that guarantees conviction (4,5)
I believe the answer is:
show trial
'that guarantees conviction' is the definition.
(in a show trial, conviction is predetermined)
'demonstrate test' is the wordplay.
'demonstrate' becomes 'show' (synonyms).
'test' becomes 'trial' (synonyms).
'show'+'trial'='SHOW TRIAL'
(Other definitions for show trial that I've seen before include "Mock public examination" , "This process manipulated" , "legal spectacular" , "Court proceedings where the guilty decision has been decided in advance" , "Travesty of justice" .)