By providing parking for royalty (3)
I believe the answer is:
per
'by' is the definition.
(I've seen this in another clue)
'parking for royalty' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'for' is a charade indicator (letters next to each other).
'royalty' becomes 'ER' (abbreviation for Elizabeth Regina).
'p'+'er'='PER'
'providing' acts as a link.
(Other definitions for per that I've seen before include "For each or every" , "Preposition indicating by means of" , "Preposition meaning for each" , "According to" .)