Breathing tube (7)
I believe the answer is:
snorkel
'breathing tube' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for snorkel that I've seen before include "Breathing pipe for swimmers" , "Bent tube for a swimmer's mouth" , "Breathing aid for a swimmer" , "Diver's breathing tube" , "Breathing tube for under-water swimmer" .)