Well-briefed army leader fighting English (5)
I believe the answer is:
aware
'well-briefed' is the definition.
'army leader fighting english' is the wordplay.
'leader' indicates taking the first letters.
'fighting' becomes 'war'.
'english' becomes 'e' (abbreviation).
The initial letter of 'army' is 'a'.
'a'+'war'+'e'='AWARE'
(Other definitions for aware that I've seen before include "Conscious or having knowledge of" , "Acquainted with or conscious of" , "Cognisant, mindful" , "Informed; conscious" , "fly" .)