Flirting beginning to destroy partnership (9)
I believe the answer is:
dalliance
'flirting' is the definition.
('dalliance' can be a synonym of 'flirting')
'beginning to destroy partnership' is the wordplay.
'beginning to' suggests taking the first letters.
'partnership' becomes 'alliance' (I've seen this before).
The first letter of 'destroy' is 'd'.
'd'+'alliance'='DALLIANCE'
(Other definitions for dalliance that I've seen before include "Casual friendship" , "amorous play" , "Casual relationship" , "Affair" , "Dawdling" .)