Planet needing change of heart (5)
I believe the answer is:
earth
'planet' is the definition.
(I've seen this before)
'change of heart' is the wordplay.
'change of' indicates an anagram.
'heart' with letters rearranged gives 'EARTH'.
'needing' acts as a link.
(Other definitions for earth that I've seen before include "Fox's underground lair" , "electrical safety precaution" , "Human world" , "cost of which is very high" , "Huge cost" .)