The most common and most likely answer to this crossword clue is the 5 letter word GRUEL
I've seen this crossword clue in The Times
G
R
U
E
L
More clues leading to the result GRUEL
- Good rule for processing poor food
- Poor serving? Set to clinch game
- Good game against the Spanish but it was disappointingfare
- Good rule to mix up thin porridge
- Food not finished by setter dismissing starter as unappetising fare
- Insubstantial meal, not half horrible, getting left
- Good rule to be broken for porridge
- Starving, ultimately regret having left thin watery stuff
- Porridge in heartless gaol, full of regret
- Thin food not half horrible left
Buddy explains!

The straight part
We know that "hardly appetising fare" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hardly appetising fare = gruel.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 GRUEL mean?
GRUEL noun- A thin, watery porridge, formerly eaten primarily by the poor and the ill.
- To exhaust; use up; disable; to punish.