Break down chemically (7)
I believe the answer is:
degrade
'break down chemically' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for degrade that I've seen before include "Disgrace, lower in status" , "Demean, cheapen" , "Decompose chemically" , "Break down; debase" , "reduce quality" .)