English king and queen fall from grace (3)
I believe the answer is:
err
'fall from grace' is the definition.
(I've seen this in another clue)
'english king and queen' is the wordplay.
'english' becomes 'E' (abbreviation).
'king' becomes 'r' (abbreviation of Latin rex).
'and' says to put letters next to each other.
'queen' becomes 'r' (abbreviation for regina, e.g. in ER ).
'r' put after 'r' is 'rr'.
'e'+'rr'='ERR'
(Other definitions for err that I've seen before include "'To . . . is human, to forgive divine'" , "Make a wrong move" , "Deviate" , "Make mistake in Canberra" , "Go astray" .)