Arrange part of Sachertorte, crumbling (11)
I believe the answer is:
orchestrate
'arrange' is the definition.
(I've seen this before)
'sachertorte crumbling' is the wordplay.
'crumbling' indicates an anagram.
'sachertorte' anagrammed gives 'ORCHESTRATE'.
'part of' is the link.
(Other definitions for orchestrate that I've seen before include "Compose" , "Arrange scores" , "Score (a piece)" , "Arrange (for maximum effect)" , "Organise (music)" .)