It's long and blackish, containing much carbon, maybe (5)
I believe the answer is:
carob
'it's long and blackish' is the definition.
I can't tell whether this definition defines the answer.
'much carbon maybe' is the wordplay.
'much' means to remove the last letter (much but not all of the word).
'maybe' indicates anagramming the letters.
'carbon' with its last letter taken off is 'carbo'.
'carbo' anagrammed gives 'CAROB'.
'containing' acts as a link.
Can you help me to learn more?
(Other definitions for carob that I've seen before include "St John's bread" , "Chocolate substitute pod" , "that one could use for chocolate" , "(Seeds yielding) powder used as a chocolate substitute" , "Saint John's bread" .)