Lower compact musical instrument (9)
I believe the answer is:
shorthorn
'lower' is the definition.
(type of cow - something which lows or moos)
'compact musical instrument' is the wordplay.
'compact' becomes 'short'.
'musical instrument' becomes 'horn' (horn is a kind of musical instrument).
'short'+'horn'='SHORTHORN'
(Other definitions for shorthorn that I've seen before include "Type of cattle" , "Neat" , "Milk supplier" , "Durham cattle breed" , "cow" .)