For starters, old British orchestra has base instrument (4)
I believe the answer is:
oboe
'instrument' is the definition.
(oboe is a kind of instrument)
'for starters old british orchestra has base' is the wordplay.
'for starters' says to take the initial letters.
'has' means one lot of letters go next to another.
'base' becomes 'e' (I've seen this in other clues).
The initial letters of 'old british orchestra' is 'obo'.
'obo'+'e'='OBOE'
(Other definitions for oboe that I've seen before include "Orchestral instrument" , "Walk with exaggerated movements" , "Orchestral piece" , "Instrument used in 1d" , "Slender double-reed instrument" .)