In the main, it's a source of upheaval (8)
I believe the answer is:
seaquake
'upheaval' is the definition.
The answer and definition can be both related to natural events as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'in the main it's a source' is the wordplay.
'in the main' becomes 'sea' (the main is an old name for the sea).
'it's a source' becomes 'quake' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sea'+'quake'='SEAQUAKE'
'of' is the link.
Can you help me to learn more?
(Other definitions for seaquake that I've seen before include "Underwater rocks" , "Tsunami" .)