Colourless like benzene, which contains carbon and hydrogen (10)
I believe the answer is:
achromatic
'colourless' is the definition.
(associated in meaning)
'which contains carbon and hydrogen' is the wordplay.
'which' becomes 'aromatic' (I can't explain this - if you can you should believe this answer much more).
'contains' means one lot of letters goes inside another.
'carbon' becomes 'C' (chemical symbol).
'and' is a charade indicator (letters next to each other).
'hydrogen' becomes 'H' (chemical symbol).
'c'+'h'='ch'
'aromatic' enclosing 'ch' is 'ACHROMATIC'.
'like benzene' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Another definition for achromatic that I've seen is " Without colour".)