Sound of keys rattling in prison (5)
I believe the answer is:
clink
I believe this is a double definition.
'sound of keys rattling' is the first definition.
(metallic sound)
'prison' is the second definition.
(the clink can informally mean prison)
'in' acts as a link.
(Other definitions for clink that I've seen before include "Ringing sound - prison" , "Sound made when hitting glass" , "Old London prison" , "1 ac [COOLER]" , "jug" .)