Leaving after the first aristocrat (4)
I believe the answer is:
toff
'aristocrat' is the definition.
(toff can mean an upper-class person)
'leaving after the first' is the wordplay.
'leaving' becomes 'off' ('I'm off' means 'I'm leaving').
'after' means one lot of letters go next to another.
'first' indicates taking the first letters.
The first letter of 'the' is 't'.
'off' put after 't' is 'TOFF'.
(Other definitions for toff that I've seen before include "Posh type" , "A swell" , "Posh bloke (sl)" , "Posh geezer" , "Person of the upper classes" .)