The most common and most likely answer to this crossword clue is the 10 letter word RE-EDUCATE
I've seen this crossword clue in The Times
R
E
-
E
D
U
C
A
T
E
Buddy explains!
The straight part
We know that "train again" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case train again = re-educate.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 RE-EDUCATE mean?
RE-EDUCATE verb- To educate or teach again, especially in order to remove bad practices.
- To rehabilitate.