The most common and most likely answer to this crossword clue is the 4 letter word FLEX
I've seen this crossword clue in The Times
F
L
E
X
More clues leading to the result FLEX
Buddy explains!

The straight part
We know that "Crook" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case crook = flex.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 FLEX mean?
FLEX noun- Flexibility, pliancy.
- The or an act of flexing.
- Any flexible insulated electrical wiring.
- A point of inflection.
- The act of flaunting something; something one considers impressive.
- To bend something.
- To repeatedly bend one of one's joints.
- To move part of the body using one's muscles.
- To tighten the muscles for display of size or strength.
- (by extension) To flaunt one's superiority.