Dive endlessly under water to get breathing apparatus (8)
I believe the answer is:
aqualung
'breathing apparatus' is the definition.
(aqualung is a kind of breathing apparatus)
'dive endlessly under water' is the wordplay.
'dive' becomes 'lunge' (I've seen this before).
'endlessly' means to remove the last letter.
'under' means one lot of letters go next to another.
'water' becomes 'aqua' (I've seen this before).
'lunge' with its last letter removed is 'lung'.
'lung' after 'aqua' is 'AQUALUNG'.
'to get' is the link.
(Other definitions for aqualung that I've seen before include "Underwater breathing apparatus" , "Diving equipment with cylinders of compressed air" , "Diver's breathing apparatus" , "Aid for diver" , "Form of scuba" .)