Overwhelm one sister on court (8)
I believe the answer is:
inundate
'overwhelm' is the definition.
(inundate can mean to overwhelm or overload)
'one sister on court' is the wordplay.
'one' becomes 'i' (Roman numeral).
'sister' becomes 'nun' (sister is a term of address for a nun).
'on' is a charade indicator (letters next to each other).
'court' becomes 'date' (romantically pursue).
'i'+'nun'+'date'='INUNDATE'
(Other definitions for inundate that I've seen before include "Flood (a place)" , "Flood - and unite (anag)" , "'Flood, swamp (8)'" , "Overrun" , "Flood, overwhelm" .)