Thrust using energy after breather (5)
I believe the answer is:
lunge
'thrust' is the definition.
(I know that lunge is a type of thrust)
'energy after breather' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'after' is a charade indicator (letters next to each other).
'breather' becomes 'lung' (you breathe with your lungs).
'e' put after 'lung' is 'LUNGE'.
'using' is the link.
(Other definitions for lunge that I've seen before include "Charge" , "Swordsman's thrust?" , "foil attack?" , "Thrust forward as in fencing" , "Move suddenly" .)