There's mischief in the air when this little imp becomes active! (7)

I believe the answer is:

gremlin

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(Other definitions for gremlin that I've seen before include "Mischievous creature responsible for machine faults" , "Unexplained technical problem or fault" , "Imp bedevilling plane" , "Naughty fairy" , "Imagined mischievous spirit" .)

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