Swimmer’s breathing tube (7)
I believe the answer is:
snorkel
'swimmer's breathing tube' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for snorkel that I've seen before include "diving equipment?" , "Tube supplying air to swimmer" , "Breathing aid for a swimmer" , "Breathing tube for under-water swimmer" , "tube at sea" .)