The most common and most likely answer to this crossword clue is the 7 letter word VETERAN
I've seen this crossword clue in The Daily Telegraph
V
E
T
E
R
A
N
More clues leading to the result VETERAN
- Long-serving surgeon managed to save energy
- Medic with drug rushed to old soldier
- Experienced surgeon operated on European
- Old Tommy in TA, never in trouble
- Old soldier — there could be a few neater characters
- Old soldier chopped up tree in transit?
- Long-serving animal expert near ground
- Experienced kind of doctor managed without drug
- Old soldier chopped tree in front
- Animal doctor near sick warhorse
Buddy explains!

The straight part
We know that "Old soldier" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old soldier = veteran.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 VETERAN mean?
VETERAN noun- A person with long experience of a particular activity.
- A group, animal, etc. with long experience of a particular activity.
- A person who has served in the armed forces, especially an old soldier who has seen long service; also called a war veteran to distinguish from veterans that weren't in armed conflict.
- Having had long experience, practice, or service.
- Of or relating to former members of the military armed forces, especially those who served during wartime.