Slip away when queen enters, overcome (8)
I believe the answer is:
vanquish
'overcome' is the definition.
(synonyms)
'slip away when queen enters' is the wordplay.
I cannot quite see how this works, but
'away' could be 'a' (abbreviation as used in sports fixtures) and 'a' is located in the answer.
'queen' could be 'qu' (abbreviation for queen) and 'qu' is found in the answer.
This explanation may well be incorrect...
(Other definitions for vanquish that I've seen before include "Defeat in a contest" , "Best" , "Defeat in battle" , "Come out better in a contest" , "Defeat, overcome someone" .)