The most common and most likely answer to this crossword clue is the 4 plus 7 letter word JUST DESERTS
I've seen this crossword clue in The Times
J
U
S
T
D
E
S
E
R
T
S
More clues leading to the result JUST DESERTS
Buddy explains!
The straight part
We know that "Fair punishment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fair punishment = just deserts.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.