The most common and most likely answer to this crossword clue is the 4 plus 2 plus 3 letter word HALF AN EGG
H
A
L
F
A
N
E
G
G
Buddy explains!
The straight part
We know that "Dieter's breakfast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dieter's breakfast = half an egg.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.