Substantial domestic plot — study extracted mineral (7)
I believe the answer is:
realgar
'mineral' is the definition.
(thesaurus)
'substantial domestic plot study' is the wordplay.
'substantial' becomes 'real' ('real' can be a synonym of 'substantial').
'domestic plot study' becomes 'gar' (I can't justify this - if you can you should believe this answer much more).
'real'+'gar'='REALGAR'
'extracted' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for realgar that I've seen before include "Soft red mineral" , "pigment" , "rare find in a hot mineral spring" .)