It may spoil radio reception — still! (6)
I believe the answer is:
static
This clue is a double definition.
'it may spoil radio reception' is the first definition.
(radio static)
'still' is the second definition.
(static can mean still or not moving)
(Other definitions for static that I've seen before include "Still, not moving" , "fixed" , "Motionless like certain electricity" , "Stuck in a traffic jam" , "Crackling or hissing noise caused by electrical interference" .)