The most common and most likely answer to this crossword clue is the 11 letter word VASOPRESSOR
I've seen this crossword clue in The Times
V
A
S
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P
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Buddy explains!

The straight part
We know that "it’ll raise blood pressure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’ll raise blood pressure = vasopressor.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.
Crossword Answer definitions
VASOPRESSOR noun- An agent that causes such constriction.
- Of or relating to the constriction of blood vessels, which usually causes a rise in blood pressure.