Flirtation is starting to damage marriage (9)
I believe the answer is:
dalliance
'flirtation' is the definition.
(synonyms)
'starting to damage marriage' is the wordplay.
'starting to' says to take the initial letters.
'marriage' becomes 'alliance' (I've seen this before).
The initial letter of 'damage' is 'd'.
'd'+'alliance'='DALLIANCE'
'is' is the link.
(Other definitions for dalliance that I've seen before include "Affair" , "Toying - trifling - delay" , "time-wasting action" , "Fling" , "Dawdling" .)