Nationalist not sure rioting seizes government (8)
I believe the answer is:
sturgeon
'nationalist' is the definition.
(Nicola Sturgeon of the Scottish National Party)
'not sure rioting seizes government' is the wordplay.
'rioting' indicates an anagram.
'seizes' means one lot of letters goes inside another.
'government' becomes 'g' (abbreviation for government).
'not'+'sure'='notsure'
'notsure' anagrammed gives 'stureon'.
'stureon' placed around 'g' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "Egg producer" , "swimmer" , "The fish that yields caviar" , "Fish sought for caviar" , "So urgent to find caviare-bearing fish" .)