Radio crackling (6)
I believe the answer is:
static
'radio crackling' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for static that I've seen before include "showing no signs of going anywhere" , "Inert or passive, like some electricity" , "Still, not moving" , "Inactive, motionless" , "irritation for the listener?" .)