Friend of Harry’s working under Rex (3)
I believe the answer is:
ron
'friend of harry's' is the definition.
(I've seen this in another clue)
'working under rex' is the wordplay.
'working' becomes 'on' (e.g. a machine is working or on).
'under' says to put letters next to each other (below, in a down clue).
'rex' becomes 'r' (abbreviation e.g. GR=George Rex, King George).
'on' put after 'r' is 'RON'.
(Other definitions for ron that I've seen before include "Short man's name" , "Boy's name (abbr)" , "--- Weasley, Harry Potter pal" , "L. - Hubbard, US sci-fi writer" , "Shortened man's name" .)