Panic about carbon being in short supply (6)
I believe the answer is:
scarce
'in short supply' is the definition.
(both can mean rare)
'panic about carbon' is the wordplay.
'panic' becomes 'scare' (synonyms).
'about' means one lot of letters goes inside another.
'carbon' becomes 'C' (C is the chemical symbol for carbon).
'scare' enclosing 'c' is 'SCARCE'.
'being' is the link.
(Other definitions for scarce that I've seen before include "Common? Definitely not" , "Not often come across" , "Not plentiful, hard to find" , "Rare shock Conservative getting in" , "Deficient in quantity" .)