One has absolute authority (8)
I believe the answer is:
dictator
'one has absolute authority' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dictator that I've seen before include "Arbitrary ruler" , "Absolute ruler (Great in Chaplin film)" , "All-powerful ruler" , "Tyrannical ruler" , "Top dog like Hitler, Stalin and Franco" .)