Uncle George! Niece is showing some spirit! (5)
I believe the answer is:
genie
'some spirit' is the definition.
(genie is a kind of spirit)
'uncle george niece is showing' is the wordplay.
'is showing' indicates the answer is hidden within the clue (the answer is shown in the clue).
'GENIE' is hidden in the letters of 'uncle george niece'.
(Other definitions for genie that I've seen before include "Magic spirit" , "Goblin as found in Aladdin's lamp" , "Magician residing in a lamp" , "Magical Eastern spirit with strange powers" , "means of wishfulfilment" .)