Dejected, seeing hundred others killed in battle (11)
I believe the answer is:
crestfallen
'dejected' is the definition.
('crestfallen' can be similar in meaning to 'dejected')
'hundred others killed in battle' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'others' becomes 'rest' (I've seen this before).
'killed in battle' becomes 'fallen' (I've seen this before).
'c'+'rest'+'fallen'='CRESTFALLEN'
'seeing' is the link.
(Other definitions for crestfallen that I've seen before include "Downcast" , "in a sorry condition" , "Feeling a blow to one's pride" , "Low" , "Sad, disappointed" .)