Type of battery (3,4)
I believe the answer is:
dry cell
'type of battery' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" , "Source of power" .)