Hole allowing gas or air to escape (4)
I believe the answer is:
vent
'hole allowing gas or air to escape' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for vent that I've seen before include "Broadcast" , "Slit in the back of a jacket" , "Give free expression to -- opening" , "Opening of a volcano" , "Opening for air/gas" .)