The jumper from a goblin's back? (4)
I believe the answer is:
flea
'the jumper' is the definition.
(something that jumps)
'a goblin's back?' is the wordplay.
'goblin' becomes 'elf' (I've seen this before).
'back?' says the letters should be written in reverse (I've seen 'back' mean this).
'a'+'elf'='aelf'
'aelf' reversed gives 'FLEA'.
'from' acts as a link.
(Other definitions for flea that I've seen before include "Irritating jumper?" , "Tiny circus performer?" , "Little jumper?" , "Little jumping insect" , "Jumping bloodsucker" .)