Heaven swore to come down to earth (8,4)
I believe the answer is:
promised land
'heaven' is the definition.
(Promised Land is an example)
'swore to come down to earth' is the wordplay.
'swore' becomes 'promised' (I've seen this before).
'to come down to earth' becomes 'land' (I've seen this before).
'promised'+'land'='PROMISED LAND'
(Other definitions for promised land that I've seen before include "What revolutionaries often did to peasants" , "Place of milk and honey" , "Longed-for place of expected fulfilment" , "paradise" , "Canaan - where one hopes to find happiness" .)