Overwhelm one sister with fruit (8)
I believe the answer is:
inundate
'overwhelm' is the definition.
(inundate can mean to overwhelm or overload)
'one sister with fruit' is the wordplay.
'one' becomes 'i' (Roman numeral).
'sister' becomes 'nun' (sister is a term of address for a nun).
'with' means one lot of letters go next to another.
'fruit' becomes 'date' (date is a kind of fruit).
'i'+'nun'+'date'='INUNDATE'
(Other definitions for inundate that I've seen before include "be overwhelming" , "Flood, fill completely" , "Flood, cover with water" , "Flood - and unite (anag)" , "Aunt Enid could engulf badly" .)