Really out of energy, in this court exchange (5)
I believe the answer is:
rally
'this court exchange' is the definition.
(exchange on a tennis court)
'really out of energy' is the wordplay.
'out of' suggests deleting specific letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'really' with 'e' taken away is 'RALLY'.
'in' is the link.
(Other definitions for rally that I've seen before include "Recover; mass meeting" , "Regain health" , "(Of shares) increase after a fall" , "Mass political meeting" , "Pick up" .)