The most common and most likely answer to this crossword clue is the 12 letter word INTERFERENCE
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result INTERFERENCE
Buddy explains!

The straight part
We know that "obstruction" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case obstruction = interference.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 INTERFERENCE mean?
INTERFERENCE noun- The act of interfering with something, or something that interferes.
- The illegal obstruction of an opponent in some ball games.
- An effect caused by the superposition of two systems of waves.
- A distortion on a broadcast signal due to atmospheric or other effects.
- In United States patent law, an inter partes proceeding to determine the priority issues of multiple patent applications; a priority contest.
- The interruption of the line between an attacked piece and its defender by sacrificially interposing a piece.
- The situation where a person who knows two languages inappropriately transfers lexical items or structures from one to the other.