Dejected fellow killed in battle (10)
I believe the answer is:
chapfallen
'dejected' is the definition.
('chapfallen' can be similar in meaning to 'dejected')
'fellow killed in battle' is the wordplay.
'fellow' becomes 'chap' ('chap' can be a synonym of 'fellow').
'killed in battle' becomes 'fallen' (I've seen this before).
'chap'+'fallen'='CHAPFALLEN'
(Other definitions for chapfallen that I've seen before include "Dispirited" , "Dejected" .)