Foul, unacceptable TV channel (7)

I believe the answer is:

offside

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'foul' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

'unacceptable tv channel' is the wordplay.
'unacceptable' becomes 'off' (I've seen this in other clues).
'tv channel' becomes 'side' (I've seen this before).
'off'+'side'='OFFSIDE'

Can you help me to learn more?

(Other definitions for offside that I've seen before include "Dies off in illegal soccer position" , "(In a ball game) illegally positioned" , "Illegal state in football" , "Part of vehicle furthest from kerb" , "In a position where ball can't be played (football)" .)

I've seen this clue in The Independent.

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