Each cubicle is deficient in oxygen (4)
I believe the answer is:
both
'each' is the definition.
'cubicle is deficient in oxygen' is the wordplay.
'cubicle' becomes 'booth' ('booth' can be a synonym of 'cubicle').
'is deficient in' is a deletion indicator (to be deficient in something is to lack it).
'oxygen' becomes 'o' (atomic symbol for oxygen).
'booth' with 'o' removed is 'BOTH'.
(Other definitions for both that I've seen before include "The one and the other" , "couple" , "The two (things)" , "Each of two (alternatives)" , "Two considered together" .)