It's a date (5)
(Other definitions for lets that I've seen before include "Leases" , "Allows or causes to" , "we shall serve them again!" , "Permits" , "Yes" .)
'it's a date' is the definition.
(tryst is a kind of date)
This is the entire clue.
(Other definitions for tryst that I've seen before include "An archaic lovers' meeting" , "(Secret) meeting between lovers" , "Rendezvous with lover" , "meeting-place" , "Pre-arranged (secret) meeting" .)