Energy-producing US state alternatively provided Italy with carbon (9)
I believe the answer is:
calorific
'energy-producing' is the definition.
I can't tell whether this definition defines the answer.
'us state alternatively provided italy with carbon' is the wordplay.
'us state' becomes 'cal' (abbreviation for California).
'alternatively' becomes 'or'.
'provided' becomes 'if'.
'italy with' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'carbon' becomes 'c' ('C' can be a synonym of 'carbon').
'cal'+'or'+'if'+'i'+'c'='CALORIFIC'
Can you help me to learn more?
(Other definitions for calorific that I've seen before include "for heating" , "food has such value" , "Heat-generating" , "generating heat" .)