Boy finally stops dancing like Dannii's sister? (5)
I believe the answer is:
kylie
'dannii's sister?' is the definition.
(Kylie Minogue, sister of Dannii Minogue)
'boy finally stops dancing like' is the wordplay.
'finally' indicates one should take the final letters.
'stops' means one lot of letters goes inside another.
'dancing' indicates an anagram.
The last letter of 'boy' is 'y'.
'like' is an anagram of 'klie'.
'y' inserted into 'klie' is 'KYLIE'.
(Other definitions for kylie that I've seen before include "Aussie singer" , "-- Minogue, Australian singer" , "returning missile" , "Type of boomerang" , "Dannii M's older sister" .)