Endless calamity over time creates horror (8)
I believe the answer is:
distaste
'horror' is the definition.
(I have seen 'Mild horror' mean 'distaste' so perhaps 'horror' could also mean 'distaste')
'endless calamity over time' is the wordplay.
'endless' means to remove the last letter.
'calamity' becomes 'disaster' ('disaster' can be a synonym of 'calamity').
'over' indicates putting letters inside.
'time' becomes 't'.
'disaster' with its last letter removed is 'disaste'.
'disaste' enclosing 't' is 'DISTASTE'.
'creates' acts as a link.
(Other definitions for distaste that I've seen before include "Is stated (anag.)" , "Negative feeling" , "Feeling of dislike or aversion" , "Abhorrence" , "Feeling of dislike - unsavoury?" .)