The most common and most likely answer to this crossword clue is the 12 letter word INDOCTRINATE
I've seen this crossword clue in The Daily Telegraph
I
N
D
O
C
T
R
I
N
A
T
E
Buddy explains!

The straight part
We know that "coach" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case coach = indoctrinate.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 INDOCTRINATE mean?
INDOCTRINATE verb- To teach with a biased, one-sided or uncritical ideology; to brainwash.
- To teach; to instruct.