The most common and most likely answer to this crossword clue is the 6 letter word BIKING
I've seen this crossword clue in The Sunday Telegraph
B
I
K
I
N
G
More clues leading to the result BIKING
Buddy explains!

The straight part
We know that "outdoor activity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case outdoor activity = biking.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 BIKING mean?
BIKING verb- To ride a bike.
- To travel by bike.
- To transport by bicycle
- The sport of riding a bicycle.
- The practice of riding a motorcycle.