Formidable leader in government on border (4)
I believe the answer is:
grim
'formidable' is the definition.
(I've seen this before)
'leader in government on border' is the wordplay.
'leader in' says to take the initial letters.
'on' means one lot of letters go next to another.
'border' becomes 'rim' (both can mean an edge).
The initial letter of 'government' is 'g'.
'g'+'rim'='GRIM'
(Other definitions for grim that I've seen before include "Severe; horrible" , "Frightful" , "Stern or severe in appearance" , "Formidably stern" , "Dark, depressing" .)