Initially open base of each instrument (4)
I believe the answer is:
oboe
'instrument' is the definition.
(type of musical instrument)
'initially open base of each' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'open base of each' is 'OBOE'.
(Other definitions for oboe that I've seen before include "Double-reeded woodwind instrument" , "Instrument used in 1d" , "Musical instrument" , "Cor anglais in its family" , "Reed instrument" .)