on top of the world (8)
I believe the answer is:
ecstatic
'on top of the world' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ecstatic that I've seen before include "Overjoyed, over the moon" , "In religious fervour" , "Transported by joy" , "Cites act to be delighted" , "in seventh heaven" .)