The most common and most likely answer to this crossword clue is the 13 plus 4 letter word OPEN-AND-SHUT CASE
I've seen this crossword clue in Financial Times
O
P
E
N
-
A
N
D
-
S
H
U
T
C
A
S
E
More clues leading to the result OPEN-AND-SHUT CASE
Buddy explains!
The straight part
We know that "easily decided" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case easily decided = open-and-shut case.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.