Terrifying sound rocks Switzerland (7)
I believe the answer is:
screech
'terrifying sound' is the definition.
(I've seen this before)
'rocks switzerland' is the wordplay.
'rocks' becomes 'scree' (rocks on the slope of a mountain).
'switzerland' becomes 'ch' (short for Confoederatio Helvetica, its Latin name).
'scree'+'ch'='SCREECH'
(Other definitions for screech that I've seen before include "Loud harsh cry" , "Outcry" , "High pitched shout" , "Squeal" , "cry of horror" .)