The most common and most likely answer to this crossword clue is the 11 letter word REFRIGERANT
I've seen this crossword clue in The Daily Telegraph
R
E
F
R
I
G
E
R
A
N
T
More clues leading to the result REFRIGERANT
Buddy explains!
The straight part
We know that "This will cool things down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case this will cool things down = refrigerant.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 REFRIGERANT mean?
REFRIGERANT noun- A substance used in a heat cycle that undergoes a phase change between gas and liquid to allow the cooling, as in refrigerators, air conditioners, etc.
- That which makes cool or cold, such as a medicine for allaying the symptoms of fever.
- That cools or freezes; providing relief from heat or fever.