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 wordplay.
'includes' says the answer is hidden in the clue.
'CELL' is hidden in the letters of 'cancellation'.
(Other definitions for cell that I've seen before include "Place of incarceration in body unit", "celebrated Latin", "Monastery room", "Animal or plant building unit", "The basic structural unit of all organisms", "Prison accommodation", "Chamber", "Basic biological unit", "Small room for nun, monk or prisoner" and "Unit of living matter".)