The most common and most likely answer to this crossword clue is the 13 letter word UNCOORDINATED
I've seen this crossword clue in The Times
U
N
C
O
O
R
D
I
N
A
T
E
D
Buddy explains!

The straight part
We know that "clashing colours" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case clashing colours = uncoordinated.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 UNCOORDINATED mean?
UNCOORDINATED adjective- (of a project etc) Not coordinated or properly planned
- (of body movement) Lacking coordination