Overwhelm single sister with fruit (8)
I believe the answer is:
inundate
'overwhelm' is the definition.
(thesaurus)
'single sister with fruit' is the wordplay.
'single' becomes 'i' (one).
'sister' becomes 'nun' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'fruit' becomes 'date' (date is a kind of fruit).
'i'+'nun'+'date'='INUNDATE'
(Other definitions for inundate that I've seen before include "Flood, cover with water" , "Overrun" , "Overwhelm with things to be done" , "be overwhelming" , "Flood (a place)" .)