Prisoner's room has nothing but an instrument (5)
I believe the answer is:
cello
'an instrument' is the definition.
(I know that cello is a type of bowed stringed instrument)
'prisoner's room has nothing' is the wordplay.
'prisoner's room' becomes 'cell' (I've seen this before).
'has' means one lot of letters go next to another.
'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 "Bowed musical instrument" , "It's bigger than a viola, but smaller than a double bass" , "Music-maker" , "Musical instrument like large violin held between legs" , "Lloyd Webber's instrument" .)