Recover strength (5)
I believe the answer is:
rally
'recover strength' is the definition.
(to rally is to recover one's health)
This is all the clue.
(Other definitions for rally that I've seen before include "Regain one's strength" , "Recuperate in gathering" , "(Of shares) increase after a fall" , "Get better; car meet" , "Mass meeting -- revive" .)