Cast down, neither sad, remarkably (10)
I believe the answer is:
dishearten
'cast down' is the definition.
(I've seen this before)
'neither sad remarkably' is the wordplay.
'remarkably' indicates anagramming the letters.
'neither'+'sad'='neithersad'
'neithersad' with letters rearranged gives 'DISHEARTEN'.
(Other definitions for dishearten that I've seen before include "Put a damper on" , "Take away bottle" , "Cause (someone) to lose confidence" , "Bring down" , "dismay?" .)