Volatile situation (7,4)
'volatile situation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tinderbox that I've seen before include "Volatile situation" , "Old container for fire-lighting equipment" , "Volatile person" , "Old lighter" , "Situation that is a potential source of violence" .)
(Other definitions for hornets nest that I've seen before include "Stirred up it's dangerous" , "(Stir up a) violent reaction" , "Something not to be stirred up" , "hazardous situation" , "Big trouble!" .)