The most common and most likely answer to this crossword clue is the 10 letter word TRAVERSING
I've seen this crossword clue in The Guardian
T
R
A
V
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Buddy explains!

The straight part
We know that "Reaching across" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reaching across = traversing.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 TRAVERSING mean?
TRAVERSING verb- To travel across, often under difficult conditions.
- To visit all parts of; to explore thoroughly.
- To lay in a cross direction; to cross.
- (artillery) To rotate a gun around a vertical axis to bear upon a military target.
- , To climb or descend a steep hill at a wide angle (relative to the slope).
- To (make a cutting, an incline) across the gradients of a sloped face at safe rate.
- To act against; to thwart or obstruct.
- To pass over and view; to survey carefully.
- To plane in a direction across the grain of the wood.
- To deny formally.
- To use the motions of opposition or counteraction.
- A traversal.