The most common and most likely answer to this crossword clue is the 6 letter word CLOSES
C
L
O
S
E
S
More clues leading to the result CLOSES
Buddy explains!

The straight part
We know that "Seals up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seals up = closes.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 CLOSES mean?
CLOSES noun- An end or conclusion.
- The manner of shutting; the union of parts; junction.
- A grapple in wrestling.
- The conclusion of a strain of music; cadence.
- A double bar marking the end.
- (travel) The time when checkin staff will no longer accept passengers for a flight.
- (physical) To remove a gap.
- (social) To finish, to terminate.
- To come or gather around; to enclose; to encompass; to confine.
- To have a vector sum of 0; that is, to form a closed polygon.
- (chiefly Yorkshire) An enclosed field.
- (chiefly British) A street that ends in a dead end.
- A very narrow alley between two buildings, often overhung by one of the buildings above the ground floor.
- The common staircase in a tenement.
- A cathedral close.
- The interest which one may have in a piece of ground, even though it is not enclosed.