The most common and most likely answer to this crossword clue is the 6 letter word GRIEVE
I've seen this crossword clue in The Daily Telegraph
G
R
I
E
V
E
More clues leading to the result GRIEVE
- King George I, the day before becoming upset
- Mourn with girl after Mike’s fallen off stern
- Bemoan king imprisoned by soldier and never discovered?
- Rig out the second person who was keen?
- One will stand between king and first lady to mourn
- Mourn death of George, King and Emperor, the night before
- Suffer having to change rig with first mate
- Lament verge I hit
- Garlic, oddly, makes First Lady cry
- Nearly smile at lady's lament
Buddy explains!

The straight part
We know that "Lament" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lament = grieve.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 GRIEVE mean?
GRIEVE verb- To cause sorrow or distress to.
- To feel very sad about; to mourn; to sorrow for.
- To experience grief.
- To harm.
- To submit or file a grievance (about).
- A governor of a town or province.
- A manager or steward, e.g. of a farm.