Pitch in Oslo, perfect (5)
I believe the answer is:
slope
'pitch' is the definition.
('slope' can be a synonym of 'pitch')
'in oslo perfect' is the wordplay.
'in' says the answer is hidden in the clue.
'SLOPE' is hidden within 'oslo perfect'.
(Other definitions for slope that I've seen before include "bank" , "Gradient, incline" , "Slant" , "Poles for gradient" , "decamp" .)