The most common and most likely answer to this crossword clue is the 8 letter word DISTANCE
I've seen this crossword clue in Financial Times
D
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T
A
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More clues leading to the result DISTANCE
- Detective takes position in gap
- . Reserve position – supporting investigator
- Sit out during knees-up – social one’s widely observed of late
- I should return bearing reserve
- Reserve is tackling in front with ball gaining possession
- Reserve detective takes position
- It’s put out in the dance interval
- Gamble to keep man in space
- Aloofness first seen in Scandinavian that’s cold inside
- 19 Extent to which tides can flow?
Buddy explains!

The straight part
We know that "reserve" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reserve = distance.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 DISTANCE mean?
DISTANCE noun- The amount of space between two points, usually geographical points, usually (but not necessarily) measured along a straight line.
- Length or interval of time.
- The difference; the subjective measure between two quantities.
- Remoteness of place; a remote place.
- Remoteness in succession or relation.
- A space marked out in the last part of a racecourse.
- The entire amount of progress to an objective.
- A withholding of intimacy; alienation; variance.
- The remoteness or reserve which respect requires; hence, respect; ceremoniousness.
- The space measured back from the winning-post which a racehorse running in a heat must reach when the winner has covered the whole course, in order to run in the final heat.
- To move away (from) someone or something.
- To leave at a distance; to outpace, leave behind.