Dive exercises incorporating breather (6)
I believe the answer is:
plunge
'dive' is the definition.
('plunge' can be a synonym of 'dive')
'exercises incorporating breather' is the wordplay.
'exercises' becomes 'pe'.
'incorporating' means one lot of letters goes inside another.
'breather' becomes 'lung' (I've seen this before).
'pe' going around 'lung' is 'PLUNGE'.
(Other definitions for plunge that I've seen before include "Thrust into liquid" , "Thrust or dive into" , "Sudden fall" , "Take the . . . . . . - dive right in" , "Dive or swoop" .)