The most common and most likely answer to this crossword clue is the 11 letter word DETERMINING
I've seen this crossword clue in The Daily Telegraph
D
E
T
E
R
M
I
N
I
N
G
More clues leading to the result DETERMINING
Buddy explains!

The straight part
We know that "Deciding" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deciding = determining.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 DETERMINING mean?
DETERMINING verb- To set the boundaries or limits of.
- To ascertain definitely; to figure out, find out, or conclude by analyzing, calculating, or investigating.
- To fix the form or character of; to shape; to prescribe imperatively; to regulate; to settle.
- To fix the course of; to impel and direct; with a remoter object preceded by to.
- To bring to a conclusion, as a question or controversy; to settle authoritative or judicial sentence; to decide.
- To resolve on; to have a fixed intention of; also, to cause to come to a conclusion or decision; to lead.
- To define or limit by adding a differentia.
- To bring to an end; to finish.