Longing for battle, lost again (9)
I believe the answer is:
nostalgia
'longing' is the definition.
(nostalgia is a kind of longing)
'battle lost again' is the wordplay.
'battle' indicates anagramming the letters (the letters disrupted as if in battle).
'lost'+'again'='lostagain'
'lostagain' anagrammed gives 'NOSTALGIA'.
'for' acts as a link.
(Other definitions for nostalgia that I've seen before include "Sense of past perfect" , "dreaming of the past" , "Remembrance of Things Past" , "Fond memories" , "A longing for past times" .)