Rigorous testing starts to reveal necessary changes (9)
I believe the answer is:
stringent
'rigorous' is the definition.
('stringent' can be a synonym of 'rigorous')
'testing starts to reveal necessary changes' is the wordplay.
'starts to' suggests taking the first letters.
'changes' indicates an anagram.
The first letters of 'reveal necessary' is 'rn'.
'testing'+'rn'='testingrn'
'testingrn' with letters rearranged gives 'STRINGENT'.
(Other definitions for stringent that I've seen before include "Of rule, say strict and precise" , "(Rule) allowing no discretion" , "Strict, precise, exacting" , "Tough" , "Tight" .)