The most common and most likely answer to this crossword clue is the 8 letter word UNFENCED
I've seen this crossword clue in The Guardian
U
N
F
E
N
C
E
D
Buddy explains!

The straight part
We know that "Like an open field" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like an open field = unfenced.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 UNFENCED mean?
UNFENCED adjective- Not enclosed by a fence or other boundary; free to roam over a wider area.
- Without protection; defenseless.