Diver's breathing tube (7)
I believe the answer is:
snorkel
'diver's breathing tube' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for snorkel that I've seen before include "Surface breathing tube" , "diving equipment?" , "Tube supplying air to swimmer" , "Surface swimmer's breathing aid" , "Swimmer's air tube" .)