Dissertations written about caustic Republican leaving America (3,6)
I believe the answer is:
the states
'america' is the definition.
(I've seen this before)
'dissertations written about caustic republican' is the wordplay.
'dissertations' becomes 'theses' ('thesis' can be a synonym of 'dissertation').
'written about' indicates putting letters inside.
'caustic republican' becomes 'tat' (I can't justify this - if you can you should give a lot more credence to this answer).
'theses' placed around 'tat' is 'THE STATES'.
'leaving' acts as a link.
Can you help me to learn more?
(Other definitions for the states that I've seen before include "Country" , "America" .)