So-called fundamental state (5)
I believe the answer is:
maine
'state' is the definition.
(Maine is an example)
'so-called fundamental' is the wordplay.
'so-called' indicates a 'sounds like' (homophone) clue.
'fundamental' becomes 'main'.
'main' sounds like 'MAINE'.
(Other definitions for maine that I've seen before include "New England state" , "N. US Atlantic state" , "US state, capital Augusta (5)" , "somewhere in America" , "American state, capital Augusta" .)