Queen and King make a mistake (3)
I believe the answer is:
err
'make a mistake' is the definition.
(to err is to make an error)
'queen and king' is the wordplay.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'and' means one lot of letters go next to another.
'king' becomes 'r' (abbreviation of Latin rex).
'er'+'r'='ERR'
(Other definitions for err that I've seen before include "Stray from the straight and narrow" , "Make mistake in Canberra" , "Boob" , "have an affair?" , "Make mistakes" .)