The most common and most likely answer to this crossword clue is the 7 letter word POSTERN
I've seen this crossword clue in The Times
P
O
S
T
E
R
N
Buddy explains!

The straight part
We know that "gate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gate = postern.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 POSTERN mean?
POSTERN noun- A back gate, back door, side entrance, or other gateway distinct from the main entrance.
- By extension, a separate or hidden way in or out of a place, situation etc.
- A subterranean passage communicating between the parade and the main ditch, or between the ditches and the interior of the outworks.
- Situated at the rear; posterior.