Essential character (6)
'essential character' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nature that I've seen before include "Essential quality" , "mother, so-called" , "Fundamental character" , "Character - wildlife" , "Fundamental qualities" .)
(Other definitions for ethos that I've seen before include "Distinctive principles of an era, group, etc" , "General feeling" , "Spirit of a culture/age" , "Spirit (of a group, say)" , "Characteristic standards of those" .)