Groom a troop leader for battle (6)
I believe the answer is:
combat
'battle' is the definition.
('combat' can be a synonym of 'battle')
'groom a troop leader' is the wordplay.
'groom' becomes 'comb' (combing is a kind of grooming**).
'leader' says to take the initial letters.
The initial letter of 'troop' is 't'.
'comb'+'a'+'t'='COMBAT'
'for' acts as a link.
(Other definitions for combat that I've seen before include "Fight, struggle or contest" , "A fight or contest, sometimes mortal" , "Oppose" , "Fight against; fighting" , "Take action to deal with - fight" .)