Show one's in heaven (8)
I believe the answer is:
paradise
'heaven' is the definition.
(paradise is a kind of heaven)
'show one's in' is the wordplay.
'show' becomes 'parade' (parade is a kind of show**).
'one's' becomes 'is' (roman numeral).
'in' indicates putting letters inside.
'parade' enclosing 'is' is 'PARADISE'.
(Other definitions for paradise that I've seen before include "blissful place?" , "Poem by 1" , "one enjoyed before the fall" , "Idyllic place" , "heavenly destination" .)