Dive endlessly: you’ll get a breather! (4)
I believe the answer is:
lung
'a breather' is the definition.
(you breathe with your lungs)
'dive endlessly' is the wordplay.
'dive' becomes 'lunge' (I've seen this before).
'endlessly' means to remove the last letter.
'lunge' with its final letter taken away is 'LUNG'.
'you'll get' acts as a link.
(Other definitions for lung that I've seen before include "One of a pair of organs in the ribcage" , "Air bag?" , "Pulmonary organ" , "Organ used for breathing" , "Breathing apparatus" .)