Oscar regularly broke instrument (4)
I believe the answer is:
oboe
'instrument' is the definition.
(type of musical instrument)
'oscar regularly broke' is the wordplay.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'broke' are 'boe'.
'o'+'boe'='OBOE'
(Other definitions for oboe that I've seen before include "issuer of notes" , "Musical instrument" , "Double-reed wind instrument" , "Double-reeded woodwind instrument" , "Orchestral instrument" .)