Record start of diet? Count me out! (8)
I believe the answer is:
'record' is the definition.
(documenting is a kind of recording)
'start of diet? count me out' is the wordplay.
'start of' says to take the initial letters.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
The first letter of 'diet' is 'd'.
'countme' anagrammed gives 'ocument'.
(Other definitions for document that I've seen before include "Written record" , "Paper, form" , "Written statement" , "Don cut me (anag)" , "Piece of writing, of an official nature perhaps" .)