Overwhelm single sister? Go steady! (8)
I believe the answer is:
inundate
'overwhelm' is the definition.
(inundate can mean to overwhelm or overload)
'single sister? go steady' is the wordplay.
'single' becomes 'I' (one).
'sister?' becomes 'nun' (sister is a term of address for a nun).
'go steady' becomes 'date' (go steady can mean to date or go out with).
'i'+'nun'+'date'='INUNDATE'
(Other definitions for inundate that I've seen before include "Submerge or cover in water" , "Flood, cover with water" , "Overwhelm (as if) with water" , "Overwhelm, swamp" , "'Flood, swamp (8)'" .)