Cue in slang I screwed up (6)
I believe the answer is:
signal
'cue' is the definition.
(I've seen this in another clue)
'in slang i screwed up' is the wordplay.
'in' indicates the answer is hidden within the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'langis' can be found hidden inside 'slang i screwed'.
'langis' reversed gives 'SIGNAL'.
(Other definitions for signal that I've seen before include "Warning notice" , "Notable; warning sign" , "Communication that encodes a message" , "Remarkable - communication" , "Gesture to communicate" .)