A carbon free bitter (5)
I believe the answer is:
acrid
'bitter' is the definition.
(thesaurus)
'a carbon free' is the wordplay.
'carbon' becomes 'c' (C is the chemical symbol for carbon).
'free' becomes 'rid' ('rid' can be a synonym of 'free').
'a'+'c'+'rid'='ACRID'
(Other definitions for acrid that I've seen before include "Painfully pungent" , "Unpleasantly bitter" , "Sharp-smelling like smoke" , "Pungent or sharp, like smoke" , "Astringent" .)