The most common and most likely answer to this crossword clue is the 7 letter word SEALING
I've seen this crossword clue in The Sunday Telegraph
S
E
A
L
I
N
G
More clues leading to the result SEALING
Buddy explains!

The straight part
We know that "closing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case closing = sealing.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 SEALING mean?
SEALING verb- To hunt seals.
- To place a seal on (a document).
- To mark with a stamp, as an evidence of standard exactness, legal size, or merchantable quality.
- To fasten (something) so that it cannot be opened without visible damage.
- To prevent people or vehicles from crossing (something).
- To close securely to prevent leakage.
- To place in a sealed container.
- To place a notation of one's next move in a sealed envelope to be opened after an adjournment.
- To guarantee.
- To fix, as a piece of iron in a wall, with cement or plaster, etc.
- To close by means of a seal.
- To confirm or set apart as a second or additional wife.
- To tie up animals (especially cattle) in their stalls.
- Action of the verb to seal in any sense.
- Used for closing securely.
- To hunt seals.
- To place a seal on (a document).
- To mark with a stamp, as an evidence of standard exactness, legal size, or merchantable quality.
- To fasten (something) so that it cannot be opened without visible damage.
- To prevent people or vehicles from crossing (something).
- To close securely to prevent leakage.
- To place in a sealed container.
- To place a notation of one's next move in a sealed envelope to be opened after an adjournment.
- To guarantee.
- To fix, as a piece of iron in a wall, with cement or plaster, etc.
- To close by means of a seal.
- To confirm or set apart as a second or additional wife.
- To tie up animals (especially cattle) in their stalls.
- Hunting of seals (the animals).