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

The straight part
We know that "Gained control of" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gained control of = mastered.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 MASTERED mean?
MASTERED verb- To be a master.
- To become the master of; to subject to one's will, control, or authority; to conquer; to overpower; to subdue.
- To learn to a high degree of proficiency.
- To own; to possess.
- (especially of a musical performance) To make a master copy of.
- (usually with in) To earn a Master's degree.