Breather needed, then energy and thrust (5)
I believe the answer is:
lunge
'thrust' is the definition.
(I know that lunge is a type of thrust)
'breather needed then energy' is the wordplay.
'breather' becomes 'lung' (you breathe with your lungs).
'needed then' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'lung'+'e'='LUNGE'
'and' acts as a link.
(Other definitions for lunge that I've seen before include "dive" , "Fall upon" , "Sudden forward move" , "Jab, thrust" , "Sudden thrust forward" .)