Southeastern battlefield’s no open aspect (7)
I believe the answer is:
secrecy
'no open aspect' is the definition.
(being secretive rather than open)
'southeastern battlefield's' is the wordplay.
'southeastern' becomes 'SE'.
'battlefield' becomes 'Crecy' (Battle of Crecy, 14th-century battle).
'se'+'crecy'='SECRECY'
(Other definitions for secrecy that I've seen before include "Suppression of information" , "Concealment" , "Unwillingness to divulge information" , "Stealth; covertness" , "openly, without protection of this" .)