Rigorous testing starts to reveal necessary changes (9)
I believe the answer is:
Here is my best explanation:
'rigorous' is the definition.
('stringent' can be a synonym of 'rigorous')
'testing starts to reveal necessary changes' is the subsidiary indication.
'starts to' suggests taking the first letters.
'changes' is an anagram indicator.
The initial letters of 'reveal necessary' is 'rn'.
'testingrn' is an anagram of 'stringent'.
(Other definitions for stringent that I've seen before include "Harsh", "binding", "Strict, rigorous", "tightly drawn", "firm", "Strict, precise, exacting", "Rigorous", "Precise, exacting", "Demanding, inflexible" and "Uncompromising".)