The most common and most likely answer to this crossword clue is the 4 letter word DOJO
I've seen this crossword clue in The Guardian
D
O
J
O
More clues leading to the result DOJO
Buddy explains!
The straight part
We know that "combat training site" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case combat training site = dojo.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 DOJO mean?
DOJO noun- The ring, made of compacted clay, in which a sumo wrestling match is held.
- A training facility, usually led by one or more sensei; a hall or room used for such training.
- (by extension) A room or other facility used for other activities, such as meditation or software development.
- The dojo loach, Japanese weather loach, or pond loach (Misgurnus anguillicaudatus), a freshwater fish native to East Asia.