Began to upset dietitian (9)
I believe the answer is:
initiated
'began' is the definition.
(I've seen this before)
'upset dietitian' is the wordplay.
'upset' indicates an anagram.
'dietitian' is an anagram of 'INITIATED'.
'to' is the link.
(Other definitions for initiated that I've seen before include "Those admitted into the mysteries" , "Began" , "Begun" , "Started" , "Accepted into a group by way of a ritual" .)