Crumbling cold lump of earth (4)
I believe the answer is:
clod
'lump of earth' is the definition.
(I've seen this before)
'crumbling cold' is the wordplay.
'crumbling' indicates an anagram.
'cold' is an anagram of 'CLOD'.
(Other definitions for clod that I've seen before include "Idiot" , "Lump of earth or clay" , "dope?" , "Fool - bit of turf" , "Lump of mud" .)