The most common and most likely answer to this crossword clue is the 7 letter word SLIMMER
I've seen this crossword clue in The Times
S
L
I
M
M
E
R
More clues leading to the result SLIMMER
- Cook limits lunches at first for dieter
- Cook slowly, throwing in a bit of lard? Less fat
- Comparatively flimsy head of lettuce acquired by cook
- Be angry about pound as one wanting rid of corporation?
- Person on diet near state of anger after gaining pound
- Be close to anger about pounds — one wants to lose them?
- Cook, large inside, getting more slender
Buddy explains!

The straight part
We know that "dieter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dieter = slimmer.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.
What does SLIMMER mean?
SLIMMER adjective- Slender, thin.
- (of something abstract like a chance or margin) Very small, tiny.
- (rural) Bad, of questionable quality; not strongly built, flimsy.
- Sly, crafty.
- A person who is trying to become slim by dieting.