Fairy returned a little jumper (4)
I believe the answer is:
flea
'little jumper' is the definition.
(insect that jumps)
'fairy returned a' is the wordplay.
'fairy' becomes 'elf' (elf is a kind of fairy).
'returned' shows that the letters should be reversed in order.
'elf' reversed gives 'fle'.
'fle'+'a'='FLEA'
(Other definitions for flea that I've seen before include "Leaping insects" , "Small jumper in small circus" , "irritant type" , "In one's ear, it's a stinging rebuke" , "Irritating jumper?" .)