Muffle another holding a jumper (4)
I believe the answer is:
flea
'a jumper' is the definition.
(something that jumps)
'muffle another holding' is the wordplay.
'holding' says the answer is hidden in the clue.
'FLEA' is hidden in the letters of 'muffle another'.
(Other definitions for flea that I've seen before include "irritant type" , "tiny jumper" , "Leaping insects" , "Bloodsucking parasite" , "one is proverbially fit" .)