Ensure observance with part written for cello (7)
I believe the answer is:
enforce
'ensure observance' is the definition.
'part written for cello' is the wordplay.
'part' indicates the answer is hidden within the clue.
'ENFORCE' is hidden within 'written for cello'.
'with' acts as a link.
(Other definitions for enforce that I've seen before include "Give effect to" , "Apply law" , "Impose with power" , "Compel compliance or obedience" , "Compel compliance with (law)" .)