The most common and most likely answer to this crossword clue is the 8 plus 7 letter word OBSTACLE COURSES
I've seen this crossword clue in The Times
O
B
S
T
A
C
L
E
C
O
U
R
S
E
S
Buddy explains!
The straight part
We know that "training grounds" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case training grounds = obstacle courses.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.