Leaderless troops oppose battle (9)
I believe the answer is:
'battle' is the definition.
(encounter is a kind of battle)
'leaderless troops oppose' is the wordplay.
'leaderless' means to remove the first letter.
'troops' becomes 'men' (I've seen this in other clues).
'oppose' becomes 'counter' (I've seen this before).
'men' with its first letter taken off is 'en'.
(Other definitions for encounter that I've seen before include "Face" , "Meet by chance with encore nut" , "Meet by chance or enexpectedly" , "Come accross, meet (9)" , "See 19" .)