After prison, Oscar will be part of orchestra (5)
I believe the answer is:
cello
'be part of orchestra' is the definition.
(I've seen this in another clue)
'after prison oscar' is the wordplay.
'after' means one lot of letters go next to another.
'prison' becomes 'cell' (prison cell).
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'cell' next to 'o' is 'CELLO'.
'will' is the link.
(Other definitions for cello that I've seen before include "Musical instrument like large violin held between knees" , "Viola's bigger brother?" , "Music-maker" , "Large stringed instrument" , "String quartet instrument" .)