No troop leader is negative (3)
I believe the answer is:
not
'is negative' is the definition.
(I've seen this in another clue)
'no troop leader' is the wordplay.
'leader' says to take the initial letters.
The initial letter of 'troop' is 't'.
'no'+'t'='NOT'
(Other definitions for not that I've seen before include "In no way, never" , "Word of denial" , "Type of logic gate" , "logically, operation needed" , "Word of negation" .)