Chemical decomposition of cacti about which great papers may be written (6,4)
I believe the answer is:
acetic acid
'chemical' is the definition.
(I know that Acetic acid is a Pharmaceutical drug)
'decomposition of cacti about which great papers may be written' is the wordplay.
I cannot really understand how this works, but
an anagram of 'cacti' is 'ticac' which is within the answer.
'about' could be 'c' (abbreviation for 'circa') and 'c' is found within the answer.
'papers' could be 'id' (identification papers) and 'id' is located in the answer.
The remaining letters 'ae' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for acetic acid that I've seen before include "pickling stuff?" , "Component of vinegar" .)