German on edge is harsh (4)
I believe the answer is:
grim
'harsh' is the definition.
(synonyms)
'german on edge' is the wordplay.
'german' becomes 'g' (abbreviation).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'edge' becomes 'rim' (rim is a kind of edge).
'g'+'rim'='GRIM'
'is' is the link.
(Other definitions for grim that I've seen before include "Dour, forbidding" , "Very serious/gloomy" , "Very serious, gloomy" , "looking black" , "Dismal, dour" .)