Pitched until tedium's set in (6)
I believe the answer is:
tilted
'pitched' is the definition.
('tilt' can be a synonym of 'pitch')
'until tedium's set in' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TILTED' is hidden within 'until tediums set'.
(Other definitions for tilted that I've seen before include "Moved into a sloping position" , "Slanted, leaning" , "'Sloping, slanted (6)'" , "At an angle" , "Inclined, slanted" .)