Half-finished fleece by a very small jumper (4)
I believe the answer is:
flea
'very small jumper' is the definition.
(fleas can jump very high)
'half-finished fleece by a' is the wordplay.
'half finished' means to take half the letters.
'by' says to put letters next to each other.
'fleece' with half the letters taken is 'fle'.
'fle'+'a'='FLEA'
(Other definitions for flea that I've seen before include "Insect (which you don't want in your ear!)" , "In your ear, this insect is a rebuke" , "source of infestation" , "beastie?" , "Irritating jumper?" .)