The most common and most likely answer to this crossword clue is the 10 letter word WHITEBOARD
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result WHITEBOARD
Buddy explains!

The straight part
We know that "Use marker pen here" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case use marker pen here = whiteboard.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 WHITEBOARD mean?
WHITEBOARD noun- A writing board finished with a hard white material, which can be written upon using special non-permanent markers and subsequently wiped clean.
- (by extension) A collaborative tool allowing several users to write and draw on the same shared display.