Significant discovery or development (12)
I believe the answer is:
breakthrough
I believe this is a double definition.
'significant discovery' is the first definition.
(eg in an investigation)
'development' is the second definition.
(I have seen 'significant development' mean 'breakthrough' so perhaps 'development' could also mean 'breakthrough')
(Other definitions for breakthrough that I've seen before include "Find" , "Significant advance" , "(At last!) the solution, eg to scientific problem" , "Significant development leading to further progress" , "Success" .)