Flirtation, date, then marriage (9)
I believe the answer is:
dalliance
'flirtation' is the definition.
(both can mean a casual relationship)
'date then marriage' is the wordplay.
'date' becomes 'd' (abbreviation e.g. dob - date of birth).
'then' is a charade indicator (letters next to each other).
'marriage' becomes 'alliance' (I've seen this before).
'd'+'alliance'='DALLIANCE'
(Other definitions for dalliance that I've seen before include "time-wasting action" , "waste of time" , "amorous play" , "Affair" , "Fling" .)