This world is heart-breaking (5)
I believe the answer is:
earth
'this world' is the definition.
('Earth' can be a synonym of 'world')
'heart-breaking' is the wordplay.
'breaking' indicates an anagram.
'heart' with letters rearranged gives 'EARTH'.
'is' is the link.
(Other definitions for earth that I've seen before include "brownish pigment?" , "Terra firma" , "Third rock from the Sun" , "Soil; planet" , "costs so much?" .)