Finally vanquish a queen and aged king (6)
I believe the answer is:
harold
'king' is the definition.
(I've seen this before)
'finally vanquish a queen and aged' is the wordplay.
'finally' suggests the final letters.
'queen' becomes 'r' (abbreviation for regina, e.g. in ER ).
'and' is a charade indicator (letters next to each other).
'aged' becomes 'old' (similar in meaning).
The final letter of 'vanquish' is 'h'.
'h'+'a'+'r'+'old'='HAROLD'
(Other definitions for harold that I've seen before include "King of England, January-October, 1066" , "Battle of Hastings loser" , "Last Anglo-Saxon king of England" , "Man's name" , "Maybe Steptoe" .)