Waterless room for prisoner of battery (3,4)

I believe the answer is:

dry cell

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.