They study elements and their compounds (8)
I believe the answer is:
chemists
'they study elements and their compounds' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for chemists that I've seen before include "Pharmacists, say" , "They fill prescriptions by doctors" , "Drug dealers" , "They study elements and their compounds" , "They dispense prescriptions" .)