The most common and most likely answer to this crossword clue is the 9 letter word FOOTPRINT
I've seen this crossword clue in The Daily Telegraph
F
O
O
T
P
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More clues leading to the result FOOTPRINT
Buddy explains!

The straight part
We know that "an impression" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an impression = footprint.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 FOOTPRINT mean?
FOOTPRINT noun- The impression of the foot in a soft substance such as sand or snow.
- Space required by a piece of equipment.
- The amount of hard drive space required for a program.
- The audit trail left by a crashed program.
- Profession or lifestyle.
- The surface space occupied by a structure.
- A company's geographic market presence.
- The ecological impact of a human activity, machine, etc.
- Availability of a satellite from the ground.