Lover's little jumper worn by me (5)
I believe the answer is:
romeo
'lover's' is the definition.
(I've seen this before)
'little jumper worn by me' is the wordplay.
'little jumper' becomes 'roo' (short for kangaroo, something which jumps. I am not sure about the 'little' bit.).
'worn by' means one lot of letters goes inside another (I've seen this in other clues).
'roo' placed around 'me' is 'ROMEO'.
Can you help me to learn more?
(Other definitions for romeo that I've seen before include "Ladies' man" , "Amorous male" , "24's lover" , "Tragic Shakespeare hero" , "Peripheral figure" .)