Knight is up before queen (3)
I believe the answer is:
sir
'knight' is the definition.
(a knight has the title Sir)
'is up before queen' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'before' says to put letters next to each other.
'queen' becomes 'r' (abbreviation for regina, e.g. in ER ).
'is' reversed gives 'si'.
'si'+'r'='SIR'
(Other definitions for sir that I've seen before include "Baronet's title" , "Honourable title" , "Form of address for a knight" , "Way to address gentleman" , "Title given to baronet" .)