The most common and most likely answer to this crossword clue is the 9 letter word UNDERPLAY
I've seen this crossword clue in The Daily Telegraph
U
N
D
E
R
P
L
A
Y
Buddy explains!

The straight part
We know that "Fail to stress" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fail to stress = underplay.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 UNDERPLAY mean?
UNDERPLAY noun- The act of underplaying.
- To play in a subordinate, or in an inferior manner; to underact a part.
- To make something seem less important than it really is.
- To play a low card when holding a high one, in the hope of a future advantage.