Ruthless gang leader, on edge (4)
I believe the answer is:
grim
'ruthless' is the definition.
(thesaurus)
'gang leader on edge' is the wordplay.
'leader' suggests taking the first letters.
'on' is a charade indicator (letters next to each other).
'edge' becomes 'rim' (rim is a kind of edge).
The first letter of 'gang' is 'g'.
'g'+'rim'='GRIM'
(Other definitions for grim that I've seen before include "Dour, forbidding" , "Very serious, gloomy" , "Dark, depressing" , "Dour and severe" , "Serious, forbidding" .)