Separate because of significant discovery (12)
I believe the answer is:
breakthrough
'significant discovery' is the definition.
(eg in an investigation)
'separate because of' is the wordplay.
'separate' becomes 'break' (breaking is a kind of separating).
'because of' becomes 'through' ('through' can mean because of or as a result of).
'break'+'through'='BREAKTHROUGH'
(Other definitions for breakthrough that I've seen before include "Significant advance" , "Success" , "Sudden significant development" , "Important discovery" , "Important development (eg in research)" .)