Lover boy Knut sporting little jumper (5)
I believe the answer is:
romeo
'lover boy' is the definition.
(I've seen this before)
'knut sporting little jumper' is the wordplay.
'knut' becomes 'me' (where the crossword setter is 'Knut').
'sporting' means one lot of letters goes inside another (inserted letters wear or 'sport' others).
'little jumper' becomes 'roo' (short for kangaroo, something which jumps).
'me' placed into 'roo' is 'ROMEO'.
(Other definitions for romeo that I've seen before include "He's a letter read out" , "Shakespearean lover" , "Ladies' man" , "Ardent male lover" , "Don Juan" .)