Foreign nobleman and ruler on horse (6)
I believe the answer is:
junker
'foreign nobleman' is the definition.
(the Junkers were gentry in Prussia)
'ruler on horse' is the wordplay.
'ruler' becomes 'ER' (abbreviation for Elizabeth Regina).
'on' means one lot of letters go next to another.
'horse' becomes 'junk' (both can mean heroin).
'er' after 'junk' is 'JUNKER'.
'and' acts as a link.
(Other definitions for junker that I've seen before include "one having a clearout?" , "Foreign aristocrat" .)