Main component of Saturn's rings
I believe the answer is:
ice
'main component of saturn's rings' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ice that I've seen before include "Top cake" , "Constituent of 2dn" , "Cold stuff used in enticement" , "Frozen liquid" , "Decorate with frosting" .)