Room in prison to take nothing but a musical instrument (5)
I believe the answer is:
cello
'a musical instrument' is the definition.
(I know that cello is a type of bowed stringed instrument)
'room in prison to take nothing' is the wordplay.
'room in prison' becomes 'cell' (prison cell).
'to take' is a charade indicator (letters next to each other).
'nothing' becomes 'O' (looks like zero - 0).
'cell'+'o'='CELLO'
'but' acts as a link.
(Other definitions for cello that I've seen before include "Tortelier's instrument" , "Instrument in the violin family" , "Lloyd Webber's instrument" , "Musical instrument like large violin held between legs" , "music produced by this?" .)