Breather during exercise for dive (6)
I believe the answer is:
plunge
'for dive' is the definition.
(synonyms)
'breather during exercise' is the wordplay.
'breather' becomes 'lung' (you breathe with your lungs).
'during' means one lot of letters goes inside another.
'exercise' becomes 'pe' (Physical Education).
'lung' put into 'pe' is 'PLUNGE'.
(Other definitions for plunge that I've seen before include "Dive or swoop" , "A steep drop" , "Jump or dive into water" , "Dive - submerge" , "Dive or fall headlong" .)