The most common and most likely answer to this crossword clue is the 6 letter word FATHOM
I've seen this crossword clue in The Daily Telegraph
F
A
T
H
O
M
More clues leading to the result FATHOM
- Understand fine party hasn’t finished
- Understand loud social gathering hasn’t ended
- Understand family’s first party finishing early
- Get the measure of plumbers
- Understand me leaving overweight chap in France
- Understand female, mostly in some depth
- Make sense of measure for sailors
- Fahrenheit, in short, is a unit
- First to fall, in short, a downward distance
- Sound measure
Buddy explains!

The straight part
We know that "Make sense of/marine unit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case make sense of/marine unit = fathom.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 FATHOM mean?
FATHOM noun- Grasp, envelopment, control.
- (now usually nautical) An English unit of length for water depth notionally based upon the width of grown man's outstretched arms but standardized as 6 feet (about 1.8 m).
- Various similar units in other systems.
- Depth of insight, mental reach or scope.
- To encircle with outstretched arms, especially to take a measurement; to embrace.
- To measure the depth of, take a sounding of.
- To get to the bottom of; to manage to comprehend; understand (a problem etc.).