Worldly change of heart (5)
I believe the answer is:
earth
'worldly change' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'of heart' is the wordplay.
'of' indicates an anagram (the answer comes from or 'of' the letters).
'heart' is an anagram of 'EARTH'.
Can you help me to learn more?
(Other definitions for earth that I've seen before include "Huge cost" , "Land" , "World - soil" , "Lair; planet" , "dirt" .)