Horse around on instrument (4)
I believe the answer is:
gong
'instrument' is the definition.
(I know that gong is a type of percussion instrument)
'horse around on' is the wordplay.
'horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'around' indicates putting letters inside.
'gg' going around 'on' is 'GONG'.
(Other definitions for gong that I've seen before include "Dinner chime or slangy medal" , "Kind of bell sounded for dinner" , "Disc struck at meal times - medal" , "Resonant metal disc" , "Dinner signal" .)