Peter's treatment is arranged in advance (6)
I believe the answer is:
preset
'is arranged in advance' is the definition.
(I've seen this before)
'peter's treatment' is the wordplay.
'treatment' is an anagram indicator.
'peters' is an anagram of 'PRESET'.
(Other definitions for preset that I've seen before include "ready in advance" , "Pester (anag.)" , "Initially adjust" , "control on channel changer" , "Systems control" .)