Soldier changes side in heaven (8)
I believe the answer is:
paradise
'heaven' is the definition.
(I know that heaven can be written as paradise)
'soldier changes side in' is the wordplay.
'soldier' becomes 'para' (short for paratrouper).
'changes' indicates an anagram.
'in' indicates putting letters inside.
'side' is an anagram of 'dise'.
'para' put inside 'dise' is 'PARADISE'.
(Other definitions for paradise that I've seen before include "Heaven; Eden" , "perfect place" , "Utopia" , "park" , "perfectly happy state" .)