Sisters welcome assignation to support island from floods (9)
I believe the answer is:
inundates
'floods' is the definition.
('inundate' can be a synonym of 'flood')
'sisters welcome assignation to support island' is the wordplay.
'sisters' becomes 'nuns' (I've seen this before).
'welcome' means one lot of letters goes inside another (in sense of 'inviting in').
'assignation' becomes 'date' (I've seen this before).
'to support' means one lot of letters go next to another (in a down clue, some letters hold others up).
'island' becomes 'i'.
'nuns' going around 'date' is 'nundates'.
'nundates' put after 'i' is 'INUNDATES'.
'from' acts as a link.
(Another definition for inundates that I've seen is " Floods".)