One on spit where temperature famously varies (4)
I believe the answer is:
gobi
'where temperature famously varies' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'one on spit' is the wordplay.
'one' becomes 'i' (Roman numeral).
'on' is a charade indicator (letters next to each other).
'spit' becomes 'gob' (I am not sure about this - if you are sure you should believe this answer much more).
'i' after 'gob' is 'GOBI'.
Can you help me to learn more?
(Other definitions for gobi that I've seen before include "Area of sand" , "Desert in east Asia" , "Desert of central China" , "Large Far Eastern desert" , "Large desert plateau of southern Mongolia and northern China" .)