Instrument in chamber orchestra initially (5)
I believe the answer is:
cello
'instrument' is the definition.
(cello is a kind of instrument)
'chamber orchestra initially' is the wordplay.
'chamber' becomes 'cell' (I've seen this before).
'initially' indicates taking the first letters.
The initial letter of 'orchestra' is 'o'.
'cell'+'o'='CELLO'
'in' acts as a link.
(Other definitions for cello that I've seen before include "Played by Rostropovich" , "Jacqueline du Pre's instrument" , "10 [INSTRUMENT]" , "large-scale fiddle" , "String quartet instrument" .)