Found dietitian taking day off? That's unusual (8)
I believe the answer is:
initiate
'found' is the definition.
(I know that found can be written as initiate)
I can't explain the remainder of the clue.
(Other definitions for initiate that I've seen before include "starter" , "Admit, to a secret society say" , "Begin - teach" , "found" , "Cause (process) to begin" .)