Instrument hit while holding end of baton up (5,4)
I believe the answer is:
snare drum
'instrument' is the definition.
(snare drum is a kind of instrument)
'hit while holding end of baton up' is the wordplay.
'hit' becomes 'murder' (hit can mean to murder).
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'holding' means one lot of letters goes inside another.
'end of' says to take the final letters.
'up' is a reversal indicator (in down clue: letters go upwards).
The last letter of 'baton' is 'n'.
'murder'+'as'='murderas'
'murderas' enclosing 'n' is 'murderans'.
'murderans' back-to-front is 'SNARE-DRUM'.
(Other definitions for snare drum that I've seen before include "item in kitchen?" , "nine [RUFF] could come from this" , "one's beaten" , "instrument in kitchen" , "One may produce a rattling sound" .)