Carbon level trend (5)
I believe the answer is:
craze
'trend' is the definition.
(craze is a kind of trend)
'carbon level' is the wordplay.
'carbon' becomes 'C' (C is the chemical symbol for carbon).
'level' becomes 'raze' (synonyms).
'c'+'raze'='CRAZE'
(Other definitions for craze that I've seen before include "latest thing" , "Fad, short-lived enthusiasm" , "Make cracks" , "Produce fine cracks in (a surface)" , "Sudden widespread enthusiasm (for something)" .)