Caligula, for one (7)
I believe the answer is:
emperor
'caligula for one' is the definition.
(Caligula was a Roman emperor)
This is all the clue.
(Other definitions for emperor that I've seen before include "Sovereign - ruler" , "noted by Beethoven" , "Beethoven's Piano Concerto No. 5 in E-flat major" , "Absolute ruler" , "Kaiser" .)