The most common and most likely answer to this crossword clue is the 7 letter word GROUPER
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More clues leading to the result GROUPER
Buddy explains!

The straight part
We know that "submarine" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case submarine = grouper.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 GROUPER mean?
GROUPER noun- Any of various large food and game fishes of the subfamily Epinephelinae, especially the genera Epinephelus and Mycteroperca, which inhabit warm seas.
- One who groups things.