Compound -- atoms under it, I suspect (6,7)
I believe the answer is:
sodium nitrate
'compound' is the definition.
(sodium nitrate is a kind of compound)
'atoms under it i suspect' is the wordplay.
'suspect' indicates anagramming the letters (a suspicious or unusual spelling).
'atoms'+'under'+'it'+'i'='atomsunderiti'
'atomsunderiti' anagrammed gives 'SODIUM NITRATE'.
(Other definitions for sodium nitrate that I've seen before include "Fertilizer" , "Explosives may require this" , "Potentially explosive chemical" .)