The most common and most likely answer to this crossword clue is the 11 letter word SANDBAGGING
I've seen this crossword clue in Independent
S
A
N
D
B
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G
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I
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Buddy explains!
The straight part
We know that "the process of building up flood defences?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the process of building up flood defences? = sandbagging.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 SANDBAGGING mean?
SANDBAGGING verb- To construct a barrier of sandbags around.
- To strike someone with a sandbag or other object to disable or render unconscious.
- To conceal or misrepresent one's true position, potential, or intent in order to gain an advantage.