Check the advance of a sick man who returns (5)
I believe the answer is:
repel
'check' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for repel that I've seen before include "Spurn" , "Drive or force back" , "'Drive back, repulse (5)'" , "Successfully resist" , "Be impervious to" .)