Single male overcome by evil idiot (8)
I believe the answer is:
bonehead
'idiot' is the definition.
(both can mean a stupid person)
'single male overcome by evil' is the wordplay.
'single' becomes 'one' (similar in meaning).
'male' becomes 'he' (he refers to a man).
'overcome by' indicates putting letters inside.
'evil' becomes 'bad' (similar in meaning).
'one'+'he'='onehe'
'onehe' inserted into 'bad' is 'BONEHEAD'.
(Other definitions for bonehead that I've seen before include "dope" , "Stupid person with ossified brain" , "Idiot" , "Fool" , "Numbskull" .)