The most common and most likely answer to this crossword clue is the 10 letter word PROGRAMMER
I've seen this crossword clue in The Times
P
R
O
G
R
A
M
M
E
R
Buddy explains!
The straight part
We know that "Geek" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case geek = programmer.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 PROGRAMMER mean?
PROGRAMMER noun- One who writes computer programs; a software developer.
- One who decides which programs will be shown on a television station, or which songs will be played on a radio station.
- A device that installs or controls a software program in some other machine.
- A short film feature as part of a longer film program.