Back slang isn't a bit important (6)
I believe the answer is:
'important' is the definition.
(I've seen this before)
'back slang isn't a bit' is the wordplay.
'back' shows that the letters should be reversed in order.
'a bit' indicates the answer is hidden within the clue (the answer is formed by a bit of the clue).
'langis' can be found hidden inside 'slang isnt'.
'langis' back-to-front is 'SIGNAL'.
(Other definitions for signal that I've seen before include "Striking; gesture" , "Gesture, light or sound which sends a message" , "Not noise" , "Alert" , "(Traffic) light" .)