Begin with famous aristocrat (5,3)
I believe the answer is:
start off
'begin' is the definition.
(I've seen this before)
'with famous aristocrat' is the wordplay.
'with' is a charade indicator (letters next to each other).
'famous' becomes 'star' (as in an all-star cast).
'aristocrat' becomes 'toff' (toff can mean an upper-class person).
'star'+'toff' is 'START-OFF'.
(Other definitions for start off that I've seen before include "get to work" , "Begin" , "Initiate" , "Make first move" .)