The most common and most likely answer to this crossword clue is the 8 letter word IDEATION
I've seen this crossword clue in The Guardian
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More clues leading to the result IDEATION
Buddy explains!

The straight part
We know that "mental power/mental power" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mental power/mental power = ideation.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 IDEATION mean?
IDEATION noun- The conceptualization of a mental image.
- The synthesis of ideas.