The most common and most likely answer to this crossword clue is the 8 letter word CALTRAPS
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The straight part
We know that "Obstacles to troop movement" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case obstacles to troop movement = caltraps.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.
Crossword Answer definitions
CALTRAPS noun- A small, metal object with spikes arranged so that, when thrown onto the ground, one always faces up as a threat to pedestrians, horses, and vehicles.
- The same object represented as a heraldic charge.
- The starthistle, Centaurea calcitrapa, a plant with sharp thorns.
- Any of a number of flowering plants in the family Zygophyllaceae, including several members of the genus Kallstroemia and the species Tribulus terrestris, native to warm temperate and tropical regions.