Edna broke every so often: not fair! (4)
I believe the answer is:
dark
'not fair' is the definition.
(as in dark hair, as opposed to fair hair)
'edna broke every so often' is the wordplay.
'every so often' means one should take alternating letters.
'edna'+'broke'='ednabroke'
The alternate letters of 'ednabroke' are 'DARK'.
(Other definitions for dark that I've seen before include "sinister and > bleak" , "Sinister - evening" , "Unlit" , "Obscure - sinister" , "Lacking light" .)