A brief affair could end a marriage (9)
I believe the answer is:
dalliance
'a brief affair' is the definition.
'dalliance' can be an answer for 'affair' (dalliance can mean a romantic affair). I am not sure about the 'a brief' bit.
'could end a marriage' is the wordplay.
'end' indicates one should take the final letters.
'a marriage' becomes 'alliance' (I've seen this before).
The final letter of 'could' is 'd'.
'd'+'alliance'='DALLIANCE'
Can you help me to learn more?
(Other definitions for dalliance that I've seen before include "Toying - trifling - delay" , "Flirting" , "Playing" , "amorous play" , "time-wasting action" .)