Investigators seizing stimulant, source of major upheavals (9)
I believe the answer is:
tectonics
'source of major upheavals' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'investigators seizing stimulant' is the wordplay.
'investigators' becomes 'tecs' ('tec' is short for detective).
'seizing' is an insertion indicator (inserted letters are seized).
'stimulant' becomes 'tonic' (I've seen this before).
'tecs' placed around 'tonic' is 'TECTONICS'.
Can you help me to learn more?
(Other definitions for tectonics that I've seen before include "Processes in the earth's crust" , "Moving plates" , "scientific study of buildings" , "Structural geology" .)