Waterless room for prisoner of battery (3,4)
I believe the answer is:
dry cell
'prisoner of battery' is the definition.
'dry cell' can be an answer for 'battery' (I've seen this before). I'm unsure of the 'prisoner of' bit.
'waterless room' is the wordplay.
'waterless' becomes 'dry' (similar in meaning).
'room' becomes 'cell' (cell is a kind of room).
'dry'+'cell'='DRY-CELL'
'for' is the link.
Can you help me to learn more?
(Other definitions for dry cell that I've seen before include "a prisoner's more comfortable with" , "Source of energy" , "Kind of battery used in smaller domestic appliances" , "Type of battery" , "Healthy accommodation for a prisoner" .)