The most common and most likely answer to this crossword clue is the 10 letter word SUBCULTURE
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Buddy explains!
The straight part
We know that "Group/customs in U-boats" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case group/customs in u-boats = subculture.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 SUBCULTURE mean?
SUBCULTURE noun- A portion of a culture distinguished by its customs or other features.
- A culture made by transferring microorganisms from a previous culture to a fresh growth medium
- To transfer (microorganisms) to a fresh growth medium in order to start a new culture