The most common and most likely answer to this crossword clue is the 11 letter word INTRENCHING
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Buddy explains!
The straight part
We know that "putting in military defence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case putting in military defence = intrenching.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 INTRENCHING mean?
INTRENCHING verb- To dig or excavate a trench; to trench.
- To surround or provide with a trench, especially for defense; to dig in.
- To establish a substantial position in business, politics, etc.
- To invade; to encroach; to infringe or trespass; to enter on, and take possession of, that which belongs to another; usually followed by on or upon.
- To cut in; to furrow; to make trenches in or upon.