The most common and most likely answer to this crossword clue is the 7 letter word RUNWAYS
I've seen this crossword clue in The Daily Telegraph
R
U
N
W
A
Y
S
Buddy explains!
The straight part
We know that "Trails" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trails = runways.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 RUNWAYS mean?
RUNWAYS noun- A defined, narrow section of land or an artificial structure used for access.
- The usual path taken by deer or other wild animals, such as from a forest to a water source.
- A narrow walkway (often on a platform) extending from a stage on which people walk, especially one used by models during fashion shows.
- In javelin, long jump, and similar events: a short track along which athletes can accelerate themselves for their jumps or throws.
- A section of land, usually paved, for airplanes to land on or take off from.
- Hence, the number of months that a startup company can operate on its cash reserves.
- A stream bed.