One sister needs to go steady with flood (8)
I believe the answer is:
inundate
'flood' is the definition.
(I know that flood can be written as inundate)
'one sister needs to go steady' is the wordplay.
'one' becomes 'i' (Roman numeral).
'sister' becomes 'nun' (sister is a term of address for a nun).
'needs' says to put letters next to each other.
'to go steady' becomes 'date' (go steady can mean to date or go out with).
'i'+'nun'+'date'='INUNDATE'
'with' is the link.
(Other definitions for inundate that I've seen before include "Flood, cover with water" , "Overrun" , "Flood, fill completely" , "Flood (a place)" , "'Flood, swamp (8)'" .)