Leader is between king and a queen (6)
I believe the answer is:
kaiser
'leader' is the definition.
'is between king and a queen' is the wordplay.
'between' indicates putting letters inside.
'king' becomes 'k' (abbreviation for king in chess).
'and' means one lot of letters go next to another.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'k'+'a'+'er'='kaer'
'is' inserted within 'kaer' is 'KAISER'.
(Other definitions for kaiser that I've seen before include "Former German emperor" , "Old German emperor like Bill" , "Austrian bigwig" , "Emperor of Austria or Germany until 1918" , "Ex-ruler of Germany" .)