Accommodation for fifteenth prisoner? It’s a big fiddle! (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Accommodation for fifteenth prisoner? It’s a big fiddle CELL O (=accommodation for fifteenth prisoner, where CELL A is for the first prisoner, CELL B for the second, etc); the “fiddle” of the definition is a violin, stringed instrument