Logo for channel that is reversing its polarity? (5)
I believe the answer is:
ident
'logo for channel' is the definition.
(type of TV graphic)
I don't understand the rest of the clue.
(Other definitions for ident that I've seen before include "not much, which shows your station" , "short film sequence" , "Broadcast indicating TV channel" , "Short BBC sequence" , "image on screen" .)