The most common and most likely answer to this crossword clue is the 9 letter word RETARDANT
I've seen this crossword clue in The Guardian
R
E
T
A
R
D
A
N
T
More clues leading to the result RETARDANT
Buddy explains!

The straight part
We know that "It delays" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it delays = retardant.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 RETARDANT mean?
RETARDANT noun- (often in combination) Something that serves to retard (slow down) the action of something
- (often in combination) Serving to retard (slow down) the action of something