Meeting to vex saint (5)
I believe the answer is:
'meeting' is the definition.
(I know that tryst is a type of appointment)
'vex saint' is the wordplay.
'vex' becomes 'try' (I've seen this in another clue).
'saint' becomes 'st' (abbreviation).
'to' is the link.
(Other definitions for tryst that I've seen before include "private meeting" , "Lump (of gold, etc)" , "Secret assignment" , "meeting-place" , "where lovers meet" .)