The most common and most likely answer to this crossword clue is the 5 letter word QUANT
I've seen this crossword clue in Independent
Q
U
A
N
T
Buddy explains!
The straight part
We know that "Mary/the geek on the equity derivatives desk" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mary/the geek on the equity derivatives desk = quant.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 QUANT mean?
QUANT noun- A quantitative analyst.
- Short for quantity.
- Short for quantifier.
- Short for quantifiable.
- Short for quantitative.
- A punting pole with a broad flange near the end to prevent it from sinking into the mud; a setting pole.
- A vertical shaft used to drive a millstone.
- To propel using a quant.