Harsh German is on edge (4)
I believe the answer is:
grim
'harsh' is the definition.
('grim' can be a synonym of 'harsh')
'german is on edge' is the wordplay.
'german' becomes 'g' (abbreviation).
'is on' is a charade indicator (letters next to each other).
'edge' becomes 'rim' (rim is a kind of edge).
'g'+'rim'='GRIM'
(Other definitions for grim that I've seen before include "Stern or severe in appearance" , "Forbidding harsh and unpleasant" , "Tight-lipped and foreboding" , "Severe; horrible" , "Stern, cruel" .)