Start bringing up characters in Fortnite (5)
I believe the answer is:
intro
'start' is the definition.
(introduction)
'up characters in fortnite' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'characters in' indicates a hidden word.
'ortni' can be found hidden inside 'fortnite'.
'ortni' backwards is 'INTRO'.
'bringing' acts as a link.
(Other definitions for intro that I've seen before include "Preamble" , "overture" , "Opening bars (abbr)" , "Song's first few bars" , "Opening instrumental passage of song" .)