Petty individual that is on express (6)
I believe the answer is:
meanie
'petty individual' is the definition.
(a mean person)
'that is on express' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'on' means one lot of letters go next to another.
'express' becomes 'mean' (mean can mean to express or communicate).
'ie' put after 'mean' is 'MEANIE'.
(Other definitions for meanie that I've seen before include "nasty person" , "Small-minded type" , "Nasty piece of work?" , "Unkind person (informal)" , "spiteful person" .)