The most common and most likely answer to this crossword clue is the 7 letter word COMPOSE
I've seen this crossword clue in The Daily Telegraph
C
O
M
P
O
S
E
More clues leading to the result COMPOSE
Buddy explains!

The straight part
We know that "Design" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case design = compose.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 COMPOSE mean?
COMPOSE verb- To make something by merging parts.
- To make up the whole; to constitute.
- To comprise.
- To construct by mental labor; to think up; particularly, to produce or create a literary or musical work.
- (sometimes reflexive) To calm; to free from agitation.
- To arrange the elements of a photograph or other picture.
- To settle (an argument, dispute etc.); to come to a settlement.
- To arrange in proper form; to reduce to order; to put in proper state or condition.
- To arrange (types) in a composing stick for printing; to typeset.