Cell willing to exterminate leaders (6)
I believe the answer is:
gamete
'cell' is the definition.
(gamete is a kind of cell)
'willing to exterminate leaders' is the wordplay.
'willing' becomes 'game' (I've seen this before).
'leaders' says to take the initial letters.
The initial letters of 'to exterminate' is 'te'.
'game'+'te'='GAMETE'
(Other definitions for gamete that I've seen before include "Egg-cell" , "An egg or sperm cell" , "Reproductive cell" , "something seedy?" , "Germ cell in reproduction" .)