Overwhelm single sister over engagement (8)
I believe the answer is:
inundate
'overwhelm' is the definition.
(I've seen this before)
'single sister over engagement' is the wordplay.
'single' becomes 'I' (one).
'sister' becomes 'nun' (sister is a term of address for a nun).
'over' means one lot of letters go next to another.
'engagement' becomes 'date' (synonyms).
'i'+'nun'+'date'='INUNDATE'
(Other definitions for inundate that I've seen before include "Overwhelm, swamp" , "Flood, fill completely" , "Flood (a place)" , "Aunt Enid could engulf badly" , "be overwhelming" .)