The jumper from a fairy's back? (4)
I believe the answer is:
flea
'the jumper' is the definition.
(something that jumps)
'a fairy's back?' is the wordplay.
'fairy' becomes 'elf' (elf is a kind of fairy).
'back?' says the letters should be written in reverse (I've seen 'back' mean this).
'a'+'elf'='aelf'
'aelf' backwards is 'FLEA'.
'from' acts as a link.
(Other definitions for flea that I've seen before include "Wingless, jumping insect" , "Little jumping insect" , "Small blood-sucking insect" , "Little jumper?" , "Circus insect?" .)