Where to buy juice on behalf of English royal household (9)
I believe the answer is:
forecourt
'where to buy juice' is the definition.
(juice can mean petrol, available at a forecourt)
'on behalf of english royal household' is the wordplay.
'on behalf of' becomes 'for' (I've seen this before).
'english' becomes 'e' (abbreviation).
'royal household' becomes 'court'.
'for'+'e'+'court'='FORECOURT'
(Other definitions for forecourt that I've seen before include "Open space" , "Area of filling station" , "Where cars may be sold" , "vehicle may have pulled up here" .)