The most common and most likely answer to this crossword clue is the 7 plus 1 plus 4 letter word RUNNING A MILE
I've seen this crossword clue in Financial Times
R
U
N
N
I
N
G
A
M
I
L
E
Buddy explains!
The straight part
We know that "Avoiding action/first under 17,19 by rail, we hear" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case avoiding action/first under 17,19 by rail, we hear = running a mile.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.