Flirting could end a marriage (9)
I believe the answer is:
dalliance
'flirting' is the definition.
(synonyms)
'could end a marriage' is the wordplay.
'end' says to take the final letters.
'a marriage' becomes 'alliance' (I've seen this before).
The last letter of 'could' is 'd'.
'd'+'alliance'='DALLIANCE'
(Other definitions for dalliance that I've seen before include "Playing" , "Casual relationship" , "amorous play" , "Flirtation" , "Fling" .)