The most common and most likely answer to this crossword clue is the 16 letter word WALKÂ INÂ THEÂ PARK
I've seen this crossword clue in The Guardian
W
A
L
K
Â
I
N
Â
T
H
E
Â
P
A
R
K
Buddy explains!
The straight part
We know that "Picnic/outdoor exercise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case picnic/outdoor exercise = walk in the park.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.