Lucille Ball holding up instrument (4)
I believe the answer is:
bell
'instrument' is the definition.
(bell is a kind of instrument)
'lucille ball holding up' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'lleb' is hidden within 'lucille ball'.
'lleb' reversed gives 'BELL'.
(Other definitions for bell that I've seen before include "Stag's cry; ringer" , "batsman Ian" , "Wide end of a trumpet" , "Buzz" , "Ringing device" .)