Say Pluto's death is an unexpected blessing (7)
I believe the answer is:
godsend
'an unexpected blessing' is the definition.
(a helpful event or person)
'say pluto's death' is the wordplay.
'say pluto's' becomes 'god's' (Pluto was a Roman god).
'death' becomes 'end' (synonyms).
'gods'+'end'='GODSEND'
'is' is the link.
(Other definitions for godsend that I've seen before include "A piece of unexpected good fortune - despatched from above?" , "The relief" , "Boon - blessing" , "Lucky find" , "Stroke of luck" .)