Flirtatious relationship ultimately destroyed marriage (9)
I believe the answer is:
dalliance
'flirtatious relationship' is the definition.
'dalliance' can be an answer for 'relationship' (I've seen this before). I am unsure of the 'flirtatious' bit.
'ultimately destroyed marriage' is the wordplay.
'ultimately' says to take the final letters.
'marriage' becomes 'alliance' (I've seen this before).
The final letter of 'destroyed' is 'd'.
'd'+'alliance'='DALLIANCE'
Can you help me to learn more?
(Other definitions for dalliance that I've seen before include "Fling" , "Flirting" , "Dawdling" , "time-wasting action" , "A casual romance" .)