Dismal days breed animosity in the end (6)
I believe the answer is:
dreary
'dismal' is the definition.
(I know that dismal can be written as dreary)
'days breed animosity in the end' is the wordplay.
'days' becomes 'd' (abbreviation for days).
'breed' becomes 'rear' (I've seen this in another clue).
'in the end' indicates one should take the final letters.
The final letter of 'animosity' is 'y'.
'd'+'rear'+'y'='DREARY'
(Other definitions for dreary that I've seen before include "Cheerless and dull" , "Dull, tedious" , "Monotonous" , "Gloomy, boring" , "Drab and depressing" .)