The most common and most likely answer to this crossword clue is the 9 letter word STRAGGLER
I've seen this crossword clue in Financial Times
S
T
R
A
G
G
L
E
R
Buddy explains!
The straight part
We know that "Lag, with rest beginning to gain ground? Taking run, he will!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lag, with rest beginning to gain ground? taking run, he will! = straggler.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 STRAGGLER mean?
STRAGGLER noun- A person who straggles, or departs from the direct or proper course, or from the company to which they belong.
- One who falls behind the rest, for example in a race.
- One who roams without any settled direction.
- A migratory animal found away from its usual range.
- A roving vagabond.
- Something that shoots, or spreads out, beyond the rest, or too far; an exuberant growth.
- Something that stands alone or by itself.