Abate, slacken (5)
I believe this is a double definition.
'abate' is the first definition.
(I know that abate can be written as let up)
'slacken' is the second definition.
(thesaurus)
(Other definitions for let up that I've seen before include "Pause, respite" , "Slack off" , "stop" , "End of crisis" , "Pause (in tiring task)" .)
(Another definition for letup that I've seen is " relief".)