Overwhelm island court holding sister (8)
I believe the answer is:
inundate
'overwhelm' is the definition.
(inundate can mean to overwhelm or overload)
'island court holding sister' is the wordplay.
'island' becomes 'i'.
'court' becomes 'date' (romantically pursue).
'holding' indicates putting letters inside.
'sister' becomes 'nun' (sister is a term of address for a nun).
'i'+'date'='idate'
'idate' going around 'nun' is 'INUNDATE'.
(Other definitions for inundate that I've seen before include "Aunt Enid could engulf badly" , "Submerge or cover in water" , "Flood (a place)" , "Overwhelm, swamp" , "Flood, cover with water" .)