The most common and most likely answer to this crossword clue is the 6 plus 9 letter word STELLA MCCARTNEY
I've seen this crossword clue in The Guardian
S
T
E
L
L
A
M
C
C
A
R
T
N
E
Y
Buddy explains!
The straight part
We know that "Designer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case designer = stella mccartney.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.