The most common and most likely answer to this crossword clue is the 8 letter word VITALISE
V
I
T
A
L
I
S
E
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The straight part
We know that "Activate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case activate = vitalise.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 VITALISE mean?
VITALISE verb- To give life to something; to animate.
- To make more vigorous; to invigorate or stimulate.