The most common and most likely answer to this crossword clue is the 13 letter word BLACKBERRYING
I've seen this crossword clue in The Guardian
B
L
A
C
K
B
E
R
R
Y
I
N
G
More clues leading to the result BLACKBERRYING
Buddy explains!

The straight part
We know that "Gleaning fruits" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gleaning fruits = blackberrying.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 BLACKBERRYING mean?
BLACKBERRYING verb- To gather or forage for blackberries.
- The act of gathering blackberries.