Old order in Dutch royal house (6)
I believe the answer is:
orange
'dutch royal house' is the definition.
(I've seen this before)
'old order' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'order' becomes 'range' (synonyms).
'o'+'range'='ORANGE'
'in' acts as a link.
(Other definitions for orange that I've seen before include "Go earn the fruit" , "Round fruit (6)" , "One's pipped" , "Name of reigning house of the Netherlands" , "Stocking-filler fruit" .)