Tennis do-over (3)
I believe the answer is:
let
'tennis do-over' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for let that I've seen before include "Allow; tennis fault" , "Allow or rent" , "Won't stop you" , "Disappoint or fail to support - ... down" , "Award (a contract)" .)