The most common and most likely answer to this crossword clue is the 13 letter word CONTERMEASURE
I've seen this crossword clue in The Sunday Telegraph
C
O
N
T
E
R
M
E
A
S
U
R
E
Buddy explains!
The straight part
We know that "a positive preventive step" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a positive preventive step = contermeasure.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.