Hell's cold: it's round and flat (4)
I believe the answer is:
disc
'it's round and flat' is the definition.
(I know that disc is circular)
'hell's cold' is the wordplay.
'hell' becomes 'dis' (I've seen this before).
'cold' becomes 'c' (eg on taps).
'dis'+'c'='DISC'
(Other definitions for disc that I've seen before include "Thin circular plate, as in CD" , "Protein-constituent acid" , "Flat round plate" , "Body without limbs" , "Layer of cartilege between vertebrae" .)