The most common and most likely answer to this crossword clue is the 5 letter word PUTTY
I've seen this crossword clue in The Guardian
P
U
T
T
Y
More clues leading to the result PUTTY
Buddy explains!

The straight part
We know that "that seals hole" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that seals hole = putty.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 PUTTY mean?
PUTTY noun- A form of cement, made from linseed oil and whiting, used to fix panes of glass.
- Any of a range of similar substances.
- A golf ball made of composition and not gutta-percha.
- To fix or fill using putty.
- Of, pertaining to, or resembling putty.
- (pronunciation spelling) pretty; purdy
- A strip of cloth wound round the leg, worn for protection or support by hikers, soldiers etc.