Old ruler a usurper, it is whispered (6)
I believe the answer is:
caesar
'old ruler' is the definition.
(Julius Caesar)
'a usurper it is whispered' is the wordplay.
I cannot really see how this works, but
'a' is found in the answer.
'it' could be 'sa' (old-fashioned abbreviation for sex appeal) and 'sa' is located in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for caesar that I've seen before include "See 12" , "Taking little bites" , "Old Roman leader" , "US comedian" , "Eg, Augustus, Julius" .)