The most common and most likely answer to this crossword clue is the 8 letter word BOUNDARY
I've seen this crossword clue in The Daily Telegraph
B
O
U
N
D
A
R
Y
More clues leading to the result BOUNDARY
- 4 possibly attached fairly evenly
- Four, perhaps, round bay when swimming
- Four obliged to join a decentralised rally
- Without touching the ground, six leap over a train line
- Four, perhaps, obliged to be on a transport network
- Tied beside a railway line
- Jog by, around border
- Line joined with a line
- Part of cricket ground constrained by a railway
- “4” caddy shouted out?
Buddy explains!

The straight part
We know that "line" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case line = boundary.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 BOUNDARY mean?
BOUNDARY noun- The dividing line or location between two areas.
- (often in the plural) The bounds, confines, or limits between immaterial things (such as one’s comfort zone, privacy, or professional sphere and the realm beyond).
- An edge or line marking an edge of the playing field.
- An event whereby the ball is struck and either touches or passes over a boundary (with or without bouncing), usually resulting in an award of 4 (four) or 6 (six) runs respectively for the batting team.
- (of a set) The set of points in the closure of a set S, not belonging to the interior of that set.