Disappear without question overcome (8)
I believe the answer is:
vanquish
'overcome' is the definition.
(synonyms)
'disappear without question' is the wordplay.
'disappear' becomes 'vanish' (synonyms).
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'question' becomes 'qu'.
'vanish' enclosing 'qu' is 'VANQUISH'.
(Other definitions for vanquish that I've seen before include "Defeat, conquer" , "Defeat, overcome someone" , "Defeat in a contest" , "Come out better in a contest" , "Defeat in battle" .)