The most common and most likely answer to this crossword clue is the 6 letter word REMORA
R
E
M
O
R
A
More clues leading to the result REMORA
Buddy explains!

The straight part
We know that "Obstacle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case obstacle = remora.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 REMORA mean?
REMORA noun- Any of various elongate fish from the family Echeneidae, the dorsal fin of which is in the form of a suction disc that can take a firm hold against the skin of larger marine animals.
- A serpent.
- A delay; a hindrance, an obstacle.
- A surgical instrument, intended to retain parts in their places.