Set up Sidney with Daisy's relative Calamity (8)
I believe the answer is:
Here is my best explanation:
'relative calamity' is the definition.
'disaster' can be an answer for 'calamity' ('disaster' can be a synonym of 'calamity'). I am not sure about the 'relative' bit.
'set up sidney with daisy's' is the subsidiary indication.
'set up sidney' becomes 'dis' (I can't justify this - if you can you should believe this answer much more).
'with' says to put letters next to each other.
'daisy' becomes 'aster' (I've seen this before).
(Other definitions for disaster that I've seen before include "Tragedy of asterids", "Great tradegy or cataclysm", "Calamity, tragedy", "Sudden calamity", "Catastrophic occurrence", "Terrible calamity", "Sudden great misfortune", "Extreme ruin or misfortune", "not a success!" and "Event resulting in great loss and misfortune".)