Cancellation includes room (4)
I believe the answer is:
Here is my best explanation:
'room' is the definition.
(cell is a kind of room)
'cancellation includes' is the subsidiary indication.
'includes' indicates the answer is hidden within the clue.
'cell' can be found hidden inside 'cancellation'.
(Other definitions for cell that I've seen before include "Prisoner's accommodation", "Prison room", "Monastic room", "Unit of living matter", "Place of incarceration in body unit", "Small compartment", "Room in prison or monastery", "Biological unit with a nucleus", "Microscopic unit of living matter" and "Animal or plant building unit".)