Minnesota ex-governor Carlson (4)
I believe the answer is:
arne
'minnesota ex-governor carlson' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for arne that I've seen before include "Rule Britannia composer, d. 1778" , "Rule, Britannia author" , "Rule Britannia's composer, d. 1778" , "Thomas --, composer of Rule, Britannia!" .)