Gamble taking breather during exercise (6)
I believe the answer is:
plunge
'gamble' is the definition.
(thesaurus)
'breather during exercise' is the wordplay.
'breather' becomes 'lung' (I've seen this before).
'during' means one lot of letters goes inside another.
'exercise' becomes 'pe' (Physical Education).
'lung' going inside 'pe' is 'PLUNGE'.
'taking' is the link.
(Other definitions for plunge that I've seen before include "Fall suddenly and uncontrollably" , "Dive or throw into" , "Dive or swoop" , "Pitch" , "Take the . . . . . . - dive right in" .)