The most common and most likely answer to this crossword clue is the 13 letter word SHILLY-SHALLY
I've seen this crossword clue in The Guardian
S
H
I
L
L
Y
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S
H
A
L
L
Y
More clues leading to the result SHILLY-SHALLY
Buddy explains!

The straight part
We know that "Be unable to decide" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be unable to decide = shilly-shally.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 SHILLY-SHALLY mean?
SHILLY-SHALLY noun- Indecision; irresolution.
- To procrastinate.
- To vacillate.
- Indecisive; wavering.