Tried, tested (6)
I believe the answer is:
proven
I believe this clue is a double definition.
'tried' is the first definition.
(similar in meaning)
'tested' is the second definition.
(similar in meaning)
(Other definitions for proven that I've seen before include "Adjective meaning demonstrated beyond doubt" , "Established with evidence" , "Conclusively demonstrated by evidence" , "Established beyond doubt" .)