Stern German on edge (4)
I believe the answer is:
grim
'stern' is the definition.
('grim' can be a synonym of 'stern')
'german on edge' is the wordplay.
'german' becomes 'g' (abbreviation).
'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 "Unrelentingly harsh" , "Very serious, gloomy" , "Harshly ironic" , "Dour and severe" , "Serious, forbidding" .)