The most common and most likely answer to this crossword clue is the 9 letter word LADDERING
I've seen this crossword clue in The Daily Telegraph
L
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More clues leading to the result LADDERING
Buddy explains!

The straight part
We know that "running" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case running = laddering.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 LADDERING mean?
LADDERING verb- To arrange or form into a shape of a ladder.
- To ascend (a building, a wall, etc.) using a ladder.
- Of a knitted garment: to develop a ladder as a result of a broken thread.
- A process of soil compaction that serves to break up clods and level a field.
- An interview technique developed in the 1960s in which the interviewer systematically drills down for greater detail at each step.
- An investment strategy where the investment is spread over multiple bonds or term deposits with rolling maturity dates.
- A regular pattern of oligonucleotide sizes on electrophoretic gels, resulting from the fragmentation of DNA strands during cell death.
- An irregular fracture pattern that arises from uneven drying.