Radio's airtime schedule essentially gets fouled up (7)

I believe the answer is:

dirtied

'fouled up' is the definition.
(to foul up is to make dirty)

I don't understand how the remainder of the clue works.

(Other definitions for dirtied that I've seen before include "Soiled" , "discoloured" .)

I've seen this clue in The Guardian.

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