The most common and most likely answer to this crossword clue is the 9 letter word PRESEASON
I've seen this crossword clue in Financial Times
P
R
E
S
E
A
S
O
N
Buddy explains!

The straight part
We know that "period" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case period = preseason.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 PRESEASON mean?
PRESEASON noun- The period before the start of a sporting season, during which players undergo training and venues are prepared
- To season in advance.