It's a world of endless hatred, simply mad (5)
I believe the answer is:
earth
'it's a world' is the definition.
('earth' can be a synonym of 'world')
'of endless hatred simply mad' is the wordplay.
'of' is an anagram indicator (the answer comes from or 'of' the letters).
'endless' means to look at the middle letters.
'simply mad' becomes 'h' (I am not sure about this - if you are sure you should believe this answer much more).
The centre of 'hatred' is 'atre'.
'atre' is an anagram of 'eart'.
'eart'+'h'='EARTH'
Can you help me to learn more?
(Other definitions for earth that I've seen before include "Ground - globe - soil" , "Land" , "Soil; electrical ground" , "our present abode" , "Fox's burrow" .)