Compound sat with inert compounds (8)
I believe the answer is:
nitrates
'compounds' is the definition.
(nitrate is a kind of compound)
'compound sat with inert' is the wordplay.
'compound' is an anagram indicator.
'with' says to put letters next to each other.
'sat' put after 'inert' is 'inertsat'.
'inertsat' with letters rearranged gives 'NITRATES'.
(Other definitions for nitrates that I've seen before include "Fertiliser salts found in train set" , "Compounds" .)