The most common and most likely answer to this crossword clue is the 8 letter word LATTICED
I've seen this crossword clue in The Times
L
A
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More clues leading to the result LATTICED
Buddy explains!

The straight part
We know that "decorative pattern" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decorative pattern = latticed.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 LATTICED mean?
LATTICED verb- To make a lattice of.
- To close, as an opening, with latticework; to furnish with a lattice.
- Provided with latticework; having a pattern of fretwork.
- Shaped or arranged like lattice.