Warm area of hard ground (6)
I believe the answer is:
hearth
'warm area' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'hard ground' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'ground' becomes 'earth' (synonyms).
'h'+'earth'='HEARTH'
'of' is the link.
Can you help me to learn more?
(Other definitions for hearth that I've seen before include "Open fireplace" , "part of furnace" , "Floor around fireplace" , "Novel" , "home circle" .)