Cryptic crossword clue
Something up one’s sleeve with knight and king in reward for working mischief 5-5

The most common and most likely answer to this crossword clue is the 11 letter word HANKY-PANKY

I've seen this crossword clue in Financial Times
H
A
N
K
Y
-
P
A
N
K
Y

Buddy explains!

The straight part
We know that "mischief" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mischief = hanky-panky.
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 HANKY-PANKY mean?

HANKY-PANKY noun
  1. Mischievous behaviour, dishonest or shady activity.
  2. Amorous behaviour, smooching, making out, necking, sexual intercourse or other sexual activity, especially if illicit.
  3. A debaucherous act; a sexual act that is considered inappropriate.

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