The most common and most likely answer to this crossword clue is the 7 letter word FARTLEK
I've seen this crossword clue in Independent
F
A
R
T
L
E
K
Buddy explains!
The straight part
We know that "running system" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case running system = fartlek.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 FARTLEK mean?
FARTLEK noun- An athletic training technique, used especially in running, in which periods of intense effort alternate with periods of less strenuous effort in a continuous workout.