At risk, prior to a breakthrough? (2,4,3)
I believe the answer is:
on thin ice
'at risk' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for on thin ice that I've seen before include "Insecure" , "like a daredevil skater?" , "in danger of a soaking?" , "sailing close to the wind?" , "Experiencing treacherous wintry conditions" .)